A crab, seen from below, infested with a rhizocephalan. The crab's right side is drawn transparent to show the branching body of the parasite. Image from 'Invertebrates,' by Brusca and Brusca.

And you will spend the next half hour with your mouth open, descending through increasing degrees of stupefied horror, as you learn exactly what you are looking at. Congratulations. You just discovered the rhizocephala: the parasitic “root headed” barnacles.

The rhizocephala are a superorder within the subclass Thecostraca, the group of crustaceans that includes all the barnacles, but their form is highly modified from the familiar acorn and gooseneck barnacles. The only visual indication we have that these creatures are related to the other barnacles is their nauplius and cyprid larvae. In the normal barnacles, these larvae find a rock, stick themselves to it, grow a shell, and spend the rest of their days filter feeding.

Rhizocephalan larvae do not settle on rocks. Instead, the females settle on other crustaceans–usually crabs, and usually on the gills, where the crab’s exoskeleton is thinnest. This is important, because soon after settling, the larva injects its soft innards into the crab. This may take place through one of the larva’s antennae, wielded like a hypodermic needle, or the larva may molt and stick itself on the crab while it develops a special stylet inside its body. When the time is right, it everts this poker into the crab and injects its guts as a worm-like body called a vermigon.

This is where it starts to get really awful. The injected mass of visceral cells, now called the “interna,” starts to grow inside the host’s fluid-filled cavities, branching out root-like through the entire body, but concentrating on the digestive tract. Somehow, the parasite keeps the host’s immune system from attacking this network of invasive threads while it grows, drawing nourishment continuously from the host’s precious bodily fluids.

Rhizocephalon externa: it is the gross-looking yellow blob highlighted under the crab's tail.

When mature, the female interna extrudes an appendage out of the host’s body. This is called the externa, and it is little more than a gently-pulsing sac of gonads, attached to the host’s body by a short stalk, emitting a trail of pheremones to attract male larvae. When these larvae arrive, they land on the externa, and inject their soft insides into it. They then migrate into special seminal receptacles and fuse with the female, becoming nothing more than soft clumps of sperm-producing tissue, themselves parasitic on the already-parasitic female. Eggs are fertilized and released, and the cycle begins again.

But wait–it gets even sicker. In many cases, the externa is extruded under the crab’s tail flap, where it would carry its own eggs. Some species of rhizocephalans actually chemically castrate the crab, and then modify its behavior–that is, perform mind control–compelling it to take care of the parasite’s gonads as if they were the crab’s own children. This works even in males, who don’t ever carry eggs, and never exhibit any of these behaviors on their own. In these cases, the crab stops growing, and never molts again. It spends the rest of its days as a kind of zombie, eating food that will be sucked out of its digestive tract by the parasite that destroyed and replaced its reproductive system.

Now aren’t you glad you know all that?

Walker, G. (2001). Introduction to the Rhizocephala (Crustacea: Cirripedia) Journal of Morphology, 249 (1), 1-8 DOI: 10.1002/jmor.1038

A short but provocative study just came out in the open-access journal PLoS ONE. As readers may or may not be aware, the Caribbean Sea has seen an invasion of lionfish over the past five to ten years. No one is sure where they came from, but they more than likely escaped from aquaria in Florida and have since been spreading. Lionfish are problematic, since they are voracious eaters of smaller fish, including juveniles of many commercially and recreationally important species, and are not restricted to just one type of habitat. They also don’t seem to have any natural predators in the Caribbean, a fact that may have something to do with their foot-long poisonous spines. Worse, we don’t even have a good idea of what controls their populations in the eastern tropical Pacific, whence they originally came.

The researchers here found suggestive evidence that large groupers might be capable of controlling lionfish populations. They compared the biomass of lionfish and groupers at 12 reef sites in the central Bahamas. Five of the sites were in a marine park where fishing is verboten, and where, as a consequence, groupers are an order of magnitude more abundant than in most of the Caribbean. They found that these protected sites had a much lower lionfish biomass.

All the standard caveats apply…correlation is not causation, small geographic area, etc. It is interesting, though, in the context of theory that suggests invasions are more likely into disturbed ecosystems. As a practical approach to lionfish management, groupers are not ideal, at least not at the moment, and the authors acknowledge as much. Based on these results, grouper densities outside of protected areas aren’t enough to have much effect on lionfish. In the absence of greatly expanded protected areas, or significant changes to grouper fisheries and management, our best bet for biocontrol is to do it ourselves. Fluffy battered lionfish, anyone?

Peter J. Mumby, Alastair R. Harborne, Daniel R. Brumbaugh (2011). Grouper as a Natural Biocontrol of Invasive Lionfish PLoS ONE, 6 (6) : doi:10.1371/journal.pone.0021510

Baird's beaked whales in the Bahamas.

One odd feature of these strandings is that the sonar, while loud, is highly unlikely to be loud enough to physically harm the whales where they encounter it. As the sound spreads outward from its source, its intensity drops off very rapidly as the finite energy in the signal is spread over more and more area. Unless the whales just happened to be within, say, 100 yards of the ship when it started transmitting, the sound would be too faint to do damage (at least as we currently understand their physiology). What’s even odder is that the whales shouldn’t even be able to hear the sonar that well—their hearing and vocalizations are centered around 40 kHz, while the sonar is around 3-8 kHz.

Since it appears unlikely that the strandings are caused by direct physical trauma, scientists began considering other explanations. One hypothesis emerged from the fact that the calls of hunting killer whales also fall in this 3-8 kHz frequency range. Could the beaked whales be mistaking the sonar for the sounds of a predator, and altering their behavior in a way that might ultimately led to their deaths?

To investigate, a team of scientists from several research institutions, NOAA, and the U.S. Navy, undertook the first real experimental study of how beaked whales respond to MFA sonar. The location was a Navy test range near the Bahamas, featuring an array of hydrophones spread over its 1124 km² (434 mi²) area. Though intended for listening to submarines, they also pick up the foraging clicks of Blainville’s beaked whales, Mesoplodon densirostris.  This set up the first part of the study. During a normal Navy sonar exercise, the researchers monitored the hydrophone array for beaked whale foraging noises. Before, during, and after the exercise, they noted which hydrophones heard whales and which didn’t.

The Navy's AUTEC hydrophone array. The red circles show which hydrophones heard whales before, during, and after a sonar exercise.

Once the noise starts, no whale calls are heard near the center of the range. At the same time, more whale clicks are heard at the edges, suggesting the whales have moved away. Over the next couple of days, they slowly return.  Before a later sonar exercise, the researchers managed to stick a satellite tracking tag on one whale, who promptly hightailed it out of  the range once the exercise began.

The scientists then moved on to phase two.  They tagged two whales with tags that not only measured their depth and location, but recorded the ambient noise. Once each whale started diving again as normal, they played one of three recordings: mid-frequency sonar, an orca call, or a random signal. Gradually, they turned up the volume to see how the whales would react. Once the volume of each playback reached a certain level, the whales’ behavior changed abruptly. They immediately went silent, halting their echolocation clicks, and began swimming towards the surface—but at a much slower rate than normal. It looked for all the world as if they were trying to be inconspicuous under the perceived threat of a predator. The threshold for these changes was about 120 decibels (dB) for the sonar and random signal, but only 100 dB for the orca calls.

What does all this mean? Well…nothing for sure, since the sample sizes in question are so small. Tagging beaked whales is hard to do, given their elusiveness and deep-diving habits. A grand total of three tagged whales and a small handful of experimental sound playbacks just isn’t enough to reach firm conclusions. But what was observed is suggestive. The beaked whales appear to actively avoid the area where the sonar is being used—there’s something about it they don’t like. And it seems that something may be related to the rough similarity between mid-frequency sonar and their predator’s hunting calls.

The sounds are not actually that similar.  But since they’re right on the lower edge of the beaked whales’ hearing, maybe they have a hard time telling the difference.  Are they getting freaked out and changing their dive behavior in a dangerous way? Maybe their apparent reluctance to surface has something to do with the gas bubble lesions seen in some stranded whales—lesions very similar to those that occur in human scuba divers when they get the bends.  (This is odd, though, since the whales ascended more slowly in the presence of noise, and decompression bubbles would be expected from ascending too fast.) At this point, we don’t know for sure.  But this study, despite the difficulties imposed by confidential military technology and highly elusive cetacean subjects, appears to have already shed a great deal of light on the problem.

Tyack PL, Zimmer WM, Moretti D, Southall BL, Claridge DE, Durban JW, Clark CW, D’Amico A, Dimarzio N, Jarvis S, McCarthy E, Morrissey R, Ward J, & Boyd IL (2011). Beaked whales respond to simulated and actual navy sonar. PloS one, 6 (3) PMID: 21423729

T.M. Cox, T.J. Ragen, A.J. REad, E. Vos, R.W. Baird, K. Balcomb, et al. (2006). Understanding the impacts of anthropogenic sound on beaked whales Journal of Cetacean Research and Management, 7 (3), 177-187

Jepson, P., Arbelo, M., Deaville, R., Patterson, I., Castro, P., Baker, J., Degollada, E., Ross, H., Herráez, P., Pocknell, A., Rodríguez, F., Howie, F., Espinosa, A., Reid, R., Jaber, J., Martin, V., Cunningham, A., & Fernández, A. (2003). Gas-bubble lesions in stranded cetaceans Nature, 425 (6958), 575-576 DOI: 10.1038/425575a

Imagine, for a moment, that you are a small planktonic crustacean floating in the tropical ocean. Your world is vast, but its physical geography at your scale is relatively simple. Light and warmth are above, dark and cold are down. Most of the information you use—to decide when to feed, to avoid predators—comes either from those diffuse light cues, or the vibrations of the water immediately around you, felt through the tiny hairs, or setae, on your antennae and body. To your tiny body, the water feels more like syrup than flowing liquid, and your world has no boundaries or hard edges…for the most part, anyway.

Because there are hard boundaries around the fringes of land. And, depending on what kind of planktonic crustacean you are, those boundaries are either your only chance at survival or a near-certain death trap. If you are a larva of an animal that lives on a reef as an adult, you need to find a suitable reef to settle on, or you will die floating in the open ocean. But if you are a holoplanktonic animal, one who lives its entire life in the water column, the reef is an alien world, full of hard rocks covered with stinging polyps and surrounded by hungry fish and other predators. You want to stay as far away from them as possible.

A brand-new study in PLoS ONE tests a hypothesis about how small planktonic crustaceans either find or avoid the Great Barrier Reef in Australia. The researchers hypothesize that they do it by listening for the sounds of the reef. This idea makes a lot of sense. Sound is pretty much the only way to gather information from far away underwater, which is why dolphins and fishermen use it to find food, navies use it to find submarines, and whales use it to communicate across ocean basins. And, as anyone who has swum, snorkeled, or dived on a reef will tell you, they are noisy places. Snapping shrimp, scraping urchins, grunting fish, and breaking waves all combine to make a distinctive background snap, crackle, and pop. Some reef fishes have been shown to respond to reef noise, and there is evidence from temperate waters that crab larvae might as well.

The researchers tested their hypothesis by placing light traps in the Great Barrier Reef lagoon, the wide strip of water in between the reef and the mainland. Light traps are relatively simple devices, with a lighted chamber open to the water through a narrow slit. Animals are attracted to the light, swim through the slit, and can’t get back out. To one of these light traps they attached an underwater sound system, playing back a looped recording of reef noise. The traps were deployed at dusk and retrieved at dawn, and the procedure was repeated on 34 nights over three months.

The results, after counting 691,000 (!) crustaceans from the traps, did in fact support the hypothesis. Crustaceans that live in the open water or emerge from soft sediments at night to feed were found preferentially in the silent traps. Crab larvae at the zoea stage, obligated to find a reef to settle on to grow into adults, were found preferentially in the trap with the reef sound playback.

Catches of crustacean taxa in light traps with and without reef noise playback. Bars above zero mean more were caught in the silent control trap. A) shows crab larvae, which want to settle on the reef. B) and C) are pelagic and night-emergent crustaceans, which want to avoid it.

This study confirms what earlier ones had suggested, that crustaceans use acoustic cues to either locate their adult habitat, or to avoid the “wall of mouths” that is a coral reef to a small zooplankter. More generally, it suggests just what a broad cross section of marine life uses sound to orient and find its way in the marine world. Human activities appear to be changing the underwater soundscape in many places around the world, and may be having effects on dolphins and whales, which are very acoustically oriented. This research raises the prospect that changes in the sonic seascape may have implications for many other animals, as well. As always, more research is needed.

Stephen D. Simpson, Andrew N. Radford, Edward J. Tickle, Mark G. Meekan, Andrew G. Jeffs (2011). Adaptive Avoidance of Reef Noise PLoS ONE : doi:10.1371/journal.pone.0016625

Montgomery JC, Jeffs A, Simpson SD, Meekan M, & Tindle C (2006). Sound as an orientation cue for the pelagic larvae of reef fishes and decapod crustaceans. Advances in marine biology, 51, 143-96 PMID: 16905427

Stanley, J., Radford, C., & Jeffs, A. (2009). Induction of settlement in crab megalopae by ambient underwater reef sound Behavioral Ecology, 21 (1), 113-120 DOI: 10.1093/beheco/arp159

From Holdredge et al. 2009.

The traditional view of marshes was that they were governed by “bottom-up” dynamics—that is, that physics, nutrients, and other non-biological factors determined where marsh plants grew or did not grow. In recent years, a number of widely-read studies have argued that top-down dynamics can have big effects on entire marine ecosystems, in some cases changing their actual physical structure.

Holdredge, Bertness, and Altieri, in a paper published last year in Conservation Biology, propose that something like this is going on in Cape Cod marshes. The culprit behind the cordgrass die-off is not, they believe, any change in nutrients, pollution, or water flow. Rather, it is the action of hungry squareback marsh crabs (Sesarma reticulatum), emerging from burrows at night and aggressively grazing back the marsh’s vegetation. The reason the crabs are eating the grass with impunity, they conjecture, is that the crabs are not being eaten by predators. Holdredge et al. tested their hypotheses using a classical experimental-ecology approach. If it is true that Sesarma crabs are eating the cordgrass, and that predation determines the abundance of crabs, then we would expect several things to be true:

First, areas with more crabs should have less cordgrass. The researchers walked along marsh channels, recording the extent of cordgrass die-back. They also placed small pitfall traps in the ground and counted how many crabs fell into them. As is turns out, die-back is indeed positively correlated with crab density. [1]

Second, we would expect grass within the reach of the crabs to be grazed more than grass the crabs cannot get to. This hypothesis was tested by transplanting clumps of grass into die-back areas and fencing some of them off to keep out crabs. Again, the data support the hypothesis: fenced-off grass grew normally, while exposed grass clumps were grazed down and clipped off.

Third, if it is true that predators are controlling the abundance of the crabs (which are in turn controlling the extent of the cordgrass), then crabs in places with more marsh die-back should be eaten less frequently. The researchers tied crabs to stakes with fishing line and left them overnight, both on Cape Cod, where die-offs are ocurring, and in Narragansett Bay, RI, where they are not. The lines were long enough that the crabs could reach existing burrows to hide. Neverthless, tethered crabs in Narragansett Bay suffered a much higher rate of predation than those on Cape Cod.

These three lines of evidence make a strong case for the crabs as culprits in cordgrass die-back on Cape Cod. The authors discuss possible reasons for the decreased predation on crabs, and settle quickly on the removal of their predators by humans. In particular they mention the tautog (Tautoga onitus). This fish eats crabs and experienced a fishing-driven population decline in the mid to late 1990′s, coinciding with accelerating marsh die back. The experiments conducted here don’t directly address the link from humans to crabs via tautog, though, and more work is necessary before pointing the finger at fishing as the definitive cause. I was also left wishing I knew more about the spatial pattern and scale of these die-backs. The researchers used aerial photos of the marsh to measure die-back through time, but didn’t show them in the paper.

On the whole, however, these guys deserve kudos for a straightforward and elegant experimental approach to determining the cause of marsh die-back, logically and clearly testing hypotheses with direct biological interpretations. Though they answer the central question (are crabs eating the cordgrass?) my interest is piqued, I’m left with a bunch of other questions I want answered as well. Are marsh die-backs are occurring all along the East Coast, or just on Cape Cod? Are they contiguous or patchy? Are they present all the time, or do they get grazed down and regrow? What is the distribution of the crab’s predators like? Hopefully, we’ll see answers to some of these soon…

For some more photos and information, check out the National Park Service’s page on crab-driven vegetation losses.

HOLDREDGE, C., BERTNESS, M., & ALTIERI, A. (2009). Role of Crab Herbivory in Die-Off of New England Salt Marshes Conservation Biology, 23 (3), 672-679 DOI: 10.1111/j.1523-1739.2008.01137.x

FOOTNOTE OF STATISTICAL GRIPES

I was going to put these in the main text, but decided that would be a little pedantic and overbearing, especially since they don’t change any of the paper’s conclusions. If you don’t care about GLMs or log-transformed count data, feel free to stop reading.

In the regression reproduced above, the authors model % cordgrass blades grazed as a linear function of the number of crabs per trap, getting a decent fit. The problem is that this model is not the right one for a response variable that is a proportion or percentage (i.e., a probability). To illustrate, plug 9 crabs/trap into the regression equation: it predicts the crabs will eat 110% of the grass blades, an impossibility. A better choice of regression model would be a logistic-link GLM with a binomial error distribution.

Also: for a couple of the ANOVAs, count data were log-transformed to meet assumptions of normality. For a discussion of why this isn’t so great, see O’Hara and Kotze 2010: Do not log-transform count data.

I just got done reading a new paper [pdf] by Ted Ames, McArthur fellow and founding director of the Penobscot East Resource Center in Stonington, Maine, which speaks directly to this question—namely, how to match the scale of fishing effort to the scale of fish stocks and populations. PERC is an organization I have a personal connection to, having interned there the summer after my Junior year in college. The paper, titled “Multispecies Coastal Shelf Recovery Plan: A Collaborative, Ecosystem-Based Approach,” proposes a new management paradigm for groundfish[1] in the eastern Gulf of Maine aimed at better matching fishery management to both fish and fishermen.

Groundfishing in the Gulf of Maine—in fact, in the entire Northwest Atlantic—has had a troubled past half-century. Stocks collapsed in the late 80s and early 90s, due to a combination of foreign and domestic overfishing, overcapitalization, ineffective management, and, more likely than not, unfavorable ocean conditions. That’s plenty of causes for a stock collapse, but a few more have become apparent since then, too.

For one, we have learned that cod and other groundfish have much more localized stock structure than was previously thought. This is a problem because NMFS sets quotas for large areas, encompassing a number groundfish sub-populations. Even if the overall quota is sustainable, it is still possible for large, mobile boats to fish down local stocks one after another. Stocks in some areas of the Gulf of Maine have seen measured recoveries, but others haven’t. Non-local fishing pressure on local stocks is probably at least partly responsible.

A second wrinkle is the effect particular types of quotas have on the fishing fleet. After several false starts, NMFS settled on a days-at-sea system, with time on the water for fishing allotted based on historical catches. Though it wasn’t the intention, this policy had the effect of shutting out smaller boats. A skipper with too few days-at-sea will find it unprofitable to fish at all, since the majority of his allotted time will be spent steaming to and from the fishing grounds. As a result of the poor recovery of the fish and the days-at-sea restriction, there is very little groundfishing in New England north of Portland, ME. East of Rockland, there is virtually none.

Ames’s proposed management solution borrows a page from another Maine fishery that had a troubled first half of the century: lobster. Maine lobster, Homarus americanus, suffered a collapse in the 1930s, driven by a canning industry that took all sizes of lobster from pre-reproduction juveniles to giant old spawners. In the wake of that collapse, local groups of lobstermen began to self-enforce limits on the number of traps and the size of “keepers.” This informal management regime was gradually folded into state regulations. In 1997, Maine explicitly divided up the coast into seven lobster management zones, yielding considerable management authority in each zone to local lobstermens’ councils. This progression of local management regimes—none of which, by the way, require much data to run—have seen consistently high landings. Just as importantly, they have let fishermen and managers see eye-to-eye when they otherwise might not.

Under the Multi-Species Coastal Shelf Recovery Plan (MSCRP), stocks within 25km of the coast would be limited to small, local boats using hooks or traps, to protect inshore stocks and habitat. An intermediate layer would be open to larger craft with somewhat looser gear restrictions, and beyond 75km, the current federal rules would apply unmodified.

I would be very interested to see a quantitative management-strategy evaluation of this scheme. From my reading and superficial knowledge of the area, it seems as though it could have a good chance at success, at least in allowing near-shore stocks and small-scale inshore fishing a chance to recover. Off the top of my head, I don’t know of any other area where a similar management strategy with nested areas of increasingly local management and effort restrictions has been implemented (if anyone knows of any, please let me know). And in the meantime, I will keep an eye on Maine’s Downeast coast, where a first step towards this idea was implemented this summer—allowing groups of fishermen from several areas to pool the new catch shares, preserving their access to the fishery. (These “sectors” are different from what Ames proposes, as they are attached to groups of fishermen, not to geographical areas.) Optimism is not high, exactly, but it is undeniably present. And when it comes to a fishery that has been more or less dead for as long as yours truly has been alive, any optimism at all is a good thing.

“Groundfish” is a catchall term for fish that live on or near the bottom. In the North Atlantic, these include cod, haddock, hake, halibut, and flounders, among others.

Ames, T. (2010). Multispecies Coastal Shelf Recovery Plan: A Collaborative, Ecosystem-Based Approach Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science, 2, 217-231 DOI: 10.1577/C09-052.1

Adding machines did exist, and some could even be made to multiply, after a fashion. They were strictly mechanical, however, and the wear and tear on various joints, machine and human, coupled with the high frequency of random inputs, discouraged us from the use of any but the most necessary statistical tools. Perhaps this is why most analytical chemists of this vintage believe that if one does the analyses right, there’s no need for statistics. This is a self-reinforcing fallacy; if you don’t do the statistics, you never discover your limitations or the limitations of your methods and the universe you are sampling.

He also talks about the analysis of water samples for salinity, in the days before CTDs could measure it in-situ with electrical conductivity:

A group of female technicians, the salinity girls, ran the Mohr-Knudsen silver nitrate titrations to a chromate endpoint for eight hours a day. Needless to say, there was a considerable turnover in this group. There was always a shortage of salinity girls, and the sample bottles kept coming in from the ships and stacking up in the hallways.

Those were different times…

The most interesting aspect of the paper is his discussion of the shift in emphasis starting to occur from ship-based sampling to automated sampling with collections of stationary instruments. My thesis research is using a stationary echosounder, so I’ve done a fair amount of thinking about the differences between the traditional ship-based approach and the more recent ocean observatory approach. I appreciated hearing about this shift from the perspective of someone who has seen the entire evolution of modern oceanography firsthand. He doesn’t discuss it in the same vocabulary I might, but he comes to essentially similar conclusions.

Wangersky’s point is that our perception of the ocean and its dynamics is hugely dependent on the tools we have available to study it. It will be a very interesting time in the coming years, as more types of sensors and instruments are deployed long-term at more locations around the world. Just by virtue of observing the ocean at a new spatio-temporal scale, we’re likely to find stuff going on that we missed before. In the old days, oceanographers generally assumed that the ocean was in a steady state. When hydrographic samples were few and far between, and you had to bring them back to shore for the “salinity girls” to analyze before you knew what they meant, this assumption was kind of necessary. These assumptions were also kind of wrong, which is the point that Stommel made in his famous 1963 paper (remember Stommel?).

As instruments, methods, and computing power have increased, these types of oversimplifications have retreated, to the point where we now embrace variability and dynamic changes, and try to understand them in and of themselves, rather than as unwanted noise on top of some supposed equilibrium or steady state that isn’t really there. Every advance in instrumentation and every expansion of the scope of our observations has yielded a new perspective on the oceans, adding on to an understanding that is slowly and gradually becoming more complete.

Peter J. Wangersky (2005). Methods of sampling and analysis and our concepts of ocean dynamics Scientia Marina, 69 (S1), 75-84 : 10.3989/scimar.2005.69s175

I’ve been thinking a lot lately about issues of scale in ecology, both because I’m taking a fascinating seminar on the topic this quarter, and because my particular research is conducive to thinking about them. “Scale” came to the fore as a topic of interest starting in the late 70′s, and is tied up with other concepts that were on the rise at that time, like chaos theory, fractal geometry, and nonlinear dynamics. An example of one of these ties is Benoit Mandelbrot’s famous question, “how long is the coast of Britain?” The answer, as he shows, depends on how long your ruler is. This turns out to be a recurring conundrum: the pattern detected by an ecological study depends very strongly on the scale of the measurements.

In 1963, fifteen years before most other people started thinking about this stuff, a short paper was published in Science that, in just four pages, managed to lay out how and why we need to consider the scale of our measurements when designing an experiment to measure oceanographic phenomena. It was written by Henry Stommel, which is not at all surprising if you know who he was. For those who don’t, Stommel was one of the original badass physical oceanographers. The kind of guy who could sit down with a pen and paper and demonstrate why the upper ocean circulates the way it is observed to. Or correctly derive the circulation of the deep ocean before it was observed. No big deal. But thanks to his 1963 paper, titled “Varieties of Oceanographic Experience,” he is cited far and wide to this day as one of the first scientists to explicitly consider the importance of scale to experimental design.

To illustrate why scale is important, take one of Stommel’s examples: say we want to measure changes in the height of the sea surface from month to month. At first glance, it would appear that only 12 measurements are necessary: go out once a month for a year, and you’re set. At second glance, however, this is ridiculous on several levels. The ocean has tides, for one. Measuring sea level once a month would give you a near-random sample of different parts of the tidal cycle, and you wouldn’t be able to detect any long-term trends. There are also shorter- and longer-term fluctuations: regular wind-driven waves, and sea-level deviations due to large oceanic eddies. To resolve sea-level changes from month to month, you will actually need to measure it every hour or so. Do it less frequently, and you will get results that are inconclusive or just plain wrong.

Choosing the right scale of measurement for your question of interest is very important. It is also not trivial, especially when the measurement is not as straightforward as the water level on a ruler nailed to the dock. Even today some people are prone to testing hypotheses using data collected at a scale inappropriate to the question. If Stommel had stopped with this message, the paper might still have found a fair number of readers. But the coup de grace was the graphic he came up with to make the message explicit:

Figure 1 from Stommel (1963).

It’s a three-dimensional surface, with time along the x-axis and distance along the y. Both are shown on a logarithmic scale, so that each tick mark is a factor of 10 larger than the one before it. Various phenomena that make the sea level go up and down are located on this surface based on their typical size and duration. The height of the surface at each point represents how much the sea level goes up or down—that is, how much energy or variability is concentrated at that space and time scale.

Gravity waves (aka wind waves, the normal ones that crash on the beach and make you seasick on boats) are typically several meters long and perturb the sea surface for a few seconds. They can therefore be placed in the lower left corner. Tides happen every 12-13 hours and affect the entire ocean basin, so they are located near the middle of the time axis, stretching from about a kilometer up to 10,000 km. And every ten thousand years or so, we hit an ice age that lowers the surface of the ocean everywhere about 100 meters, allowing humans to do things like cross the Bering Strait into North America. This figure is an elegant summary of all the different processes that perturb the sea surface, and of the spatio-temporal scales at which they all take place.

This kind of diagram (now known as a Stommel diagram) has found its way over the years into all kinds of different contexts. One direct descendent near and dear to my own heart is the one drawn up by Haury et al. in 1978. It is along similar lines, but shows variability in zooplankton abundance, not sea level. The possibilities for these diagrams are nearly endless, and not limited to the ocean, or even the natural sciences. Any system that has stuff going on over short and long distances and time spans can be clarified by sketching up a Stommel diagram. Drawing a picture like this can help make it clear how to approach your research question, and will hopefully help you avoid screwing it up by choosing the wrong scale of measurements. The hope, as Stommel put it in the last line of the paper, is to “look forward to a time when theory and observation will at last advance together in a more intimately related way.” Amen.

Stommel, H. (1963). Varieties of Oceanographic Experience: The ocean can be investigated as a hydrodynamical phenomenon as well as explored geographically Science, 139 (3555), 572-576 DOI: 10.1126/science.139.3555.572

Photo by Uwe Kils, via Wikipedia

The Euphausiids are a major group of small, shrimp-like crustaceans found worldwide in the marine plankton. Euphausia superba is probably the best-studied, and certainly the most abundant, of these species. They live all around Antarctica in the Southern Ocean, and are usually the dominant macrozooplankton grazer, occurring in vast, patchy swarms. But they are not the only one out there.

Their main competitor for the king of Antarctic plankton is a particular salp species, Salpa thompsoni. Salps are pelagic tunicates, barrel-shaped gelatinous animals that move and feed using a kind of lethargic jet propulsion, drawing water in one end of their bodies and filtering it for food particles before pushing it out the other. They also reproduce incredibly quickly, thanks to two features:

1. Their bodies are mostly water, so they do not need to actually grow that much material on their way to full size, and
2. they alternate sexual and asexual generations, with the asexual generation budding off long chains of self-feeding clones.The sexual clones then release sperm and eggs into the water, where they fertilize and grow into the next generation of asexual salps.

Together, these two facts enable salps populations to respond explosively within a few days of phytoplankton blooms. Krill, on the other hand, live several years, and spawn all at once in the early Austral spring. Their larvae will not reproduce until the next year.

Krill and salps do not tend to do well at the same time. Good years for salps tend to be bad ones for krill, and vice versa. What’s more, abundances of both are related to the extent of winter sea ice—positively for krill, and negatively for salps. This is due to a combination of their different reproductive strategies, as well as different food needs.

During the winter, krill live underneath the sea ice, and feed on ice algae, which grows on the underside of the ice pack. Well-fed krill are better prepared to produce eggs and spawn early in the season. Salps, on the other hand, can’t scrape the algae of the ice, but can respond quickly to open-water phytoplankton blooms. The upshot is that following cold winters with more sea ice, krill spawn more successfully, leading to a bigger year-class the next season. In winters with little sea ice, krill do not spawn as successfully, but salps explode as soon as the ice retreats and the spring phytoplankton bloom begins.

Over the last half-century, temperatures have been trending upwards in the Antarctic, and sea ice extent has been trending down. Along with these changes, “krill years” appear to have become less frequent. This does not augur well for the parts of the Antarctic food web that feed on krill—whales, seabirds, penguins, fish, and many others. Salps, though totally cool and more reproductively energetic than rabbits, just don’t have the crunch, tang, and oily, protein-ey goodness of Euphausia superba, the superb Antarctic Krill.

V Loeb, V Siegel, O Holm-Hansen, R Hewitt, W Fraser, W Trivelpiece, S Trivelpiece (1997). Effects of sea-ice extent and krill or salp dominance on the Antarctic food web Nature, 387, 897-900

They conclude, not surprisingly, all these cards and labels can get confusing. Ask any well-meaning seafood shopper who has tried to figure it all out. Alternatively, ask the well-meaning shopper’s marine biologist friend or family member—I’m fairly up to date on these issues, understand the principles of marine ecology, and am conversant in stock assessment science and fishery management (at least as practiced in the good old US of A). And I still need to do research to really figure out what is good to eat and what isn’t. With the proliferation of these market-based initiatives (seafood cards, certification schemes, boycotts, distributor guides) public awareness has been raised, but there is no evidence to indicate that the proliferation has led to a measurable conservation benefit for any threatened species.

They also propose several solutions to this confusion, some of which I think are better than others. The best of them, in my opinion, is directing the conservation message higher than consumers. It is reasonable to expect consumers to give a damn about whether the fish they are buying is riding the road to collapse or not. It is less reasonable to expect them to become experts in worldwide fisheries. The conservation movement would probably do better to to work with a smaller number of distributors, retailers, and chefs. As a former line cook, I can say that chefs—at least the good ones—really give a damn about the food they serve. Read this from my first chef if you need proof (warning: salty language…). There aren’t enough scientists and conservationists to build trusting relationships with every seafood eater, but we can come a bit closer with the next step up the supply chain.

Their other good ideas are eliminating harmful subsidies (that’s more a government policy one than market-driven consumer-education one) and making the seafood supply chain more transparent. The latter is, in my opinion, really important. It won’t change everyone’s behavior on it’s own, but knowing where fish came from (and for that matter, just what kind of fish it actually is…scrod, anyone?¹) is a necessary precondition to a truly sustainable system.

They have a few less-good ideas, too. IMO. One is using more negative messaging. I’m fairly skeptical that this is a good strategy in the long run. After a while, people start to tune out criticism and pessimism if it’s arriving in a constant stream. Another is setting seafood consumption targets (in addition to catch quotas). I think this is kind of analagous to an energy tax as opposed to a carbon tax: we don’t actually care if people use more energy or eat more seafood. We care if too much CO₂ enters the atmosphere, or if we catch fish faster than they can make babies. In theory, if we can effectively control the negative externality (pollution or resource mining), the rest will take care of itself. In theory, of course, all theories are correct—but I think this one is more or less right.

The fundamental truth of these consumer-education initiatives is that the ecological health of the world’s oceans can’t fit on a wallet card. Any set of information that will fit on a wallet card will necessarily leave out species, unfairly tar some responsible fisherman, give some irresponsible ones a pass, and grossly simplify an incredibly complex issue. I think they’ve done a lot of good—the demand among fisheries for eco-certification is proof that there is consumer demand for sustainable fish. But ultimately, as Jaquet and friends recognize, they aren’t the be-all and end-all. All the bike riders in Seattle won’t stop global warming, and all the wallet cards in the world won’t stop overfishing. To do that we need the really hard stuff: international agreements, transparent and traceable supply chains, and effective, adaptive, ecosystem-oriented, socially-aware, science-based management. I’m not totally on board with all their solutions, but that much I think we’d agree on.

Jacquet, J., Hocevar, J., Lai, S., Majluf, P., Pelletier, N., Pitcher, T., Sala, E., Sumaila, R., & Pauly, D. (2009). Conserving wild fish in a sea of market-based efforts Oryx, 44 (01) DOI: 10.1017/S0030605309990470

¹ A businessman comes out of Logan Airport and hails a cab. He gets in and asks the cabbie “Hey, where’s a good place around here to get scrod?” The driver looks at him in the rearview mirror and says, “Buddy, I’ve heard that question many times in many different ways. But this is the first time I’ve heard it in the past pluperfect subjunctive.” {badump-tshhhhhhhhh}