The Case of the Cod and the Seals
In the high profile, emotionally charged debate playing out in the national and international media about seals, the seal hunt and seal culls, there is one sticking point that may eventually be resolved by a Fisheries and Oceans Canada (DFO) research program.
The northwest Atlantic has the biggest populations of harp, hooded and grey seals in the world, and their numbers have increased over the past few decades. There are now estimated to be over six million seals living in these waters, and cod is definitely a part of their menu. Little wonder that there is a perception among many people that seal predation on cod is a major reason why the cod stocks are not recovering. Its proponents point out that even if a particular cod stock makes up only a small fraction of the seal diet, the sheer number of seals eating those cod is a major threat to the stock's very existence, much less its recovery. On the flip side of the coin, there are many who do not believe this to be true at all. They argue that the seals are, in fact, a positive force in any recovery of the cod stocks, because they eat other cod predators such as herring, which consume a vast number of cod eggs. Wedged in between these divergent positions is the widely accepted view in the scientific community that there are many uncertainties in our understanding of the impact that seal predation may be having on the recovery of cod and other groundfish stocks in the northwest Atlantic.
Garry Stenson, a scientist at DFO's Northwest Atlantic Fisheries Centre in St. John's, is leading a research project that is trying to pinpoint the seal's role in this ecological conundrum. One important part of the work is to determine the amount of cod, and other prey, eaten by seals. A major goal is to determine the actual proportion of cod in the diet of seals, because this is the crux of the matter, and, at present, the existing data are neither sufficient nor precise enough to convict or acquit the seal of the charges laid against it in the case of the missing cod. As Stenson says, "Ecology isn't simple, and the complexity of what appears to be a straightforward question is much greater than most people think." The immediate challenge for the research team is to put in place a suite of proven and reliable methods for collecting data and analyzing the seal's diet that, when looked at all together, provides the necessary checks and balances on the biases to be found in each method when it stands alone.
The classic way to determine an animal's diet is to study the contents of its stomach and identify the different prey based upon the hard remains present. DFO, which has been collecting seal stomachs since 1979, now has an amazing time series based on some 10,000 stomachs, and this is, without question, one of the best data sets of marine mammal consumption in the world. It is a particularly valuable resource for the study of fish such as cod that have large and robust ear bones, called otoliths, which tend to remain in the predator's stomach for a while, before being degraded by the digestive process. Otoliths not only indicate that cod are eaten, but they can also be an accurate gauge of the size and age of the consumed cod. However, while this provides a snapshot of recently eaten prey, the study of cod otoliths in a seal's stomach does have some problematic biases. For example, numbers are skewed against any animals consumed such as jellyfish, which do not have any hard parts in their bodies, or small fish that have fragile otoliths, which are not preserved in the seal's stomach. As well, the data only represent cod consumed whole, and provide little insight into partially eaten cod where the heads are not taken, such as is the case with belly biting. The luxury, though, of this database, is that it shows trends in the seal diet over a period of huge change in the ecosystem, allowing scientists to focus their research by identifying the key variables that they need to account for that may, for example, have driven the seals' need for a different diet over this period. Stenson and his colleagues are mining the data set for just this type of information with the help of students.
DFO has an unparalleled data set based on the analysis of 10,000 seal stomachs, collected back to the late 1970s. (Photo: DFO, Science)
Another type of insight into the seal's diet can be found through the analysis of fatty acid signatures. Different prey species have specific fatty acid signatures that can be detected in a sample of seal blubber, and their relative proportions have been proven to accurately reflect the proportion of different prey consumed by the seal. This method is being pioneered at Dalhousie University in conjunction with DFO, and it has already provided a new understanding of the seal's diet. For example, it has shown that harp seals eat many more small invertebrates than previously thought, while hooded seals consume fish species not seen among the stomach hard parts. As with most things in their early days, this new method still needs finetuning. Top of the list is the need to build a comprehensive reference library of the fatty acid signatures of all prey species, as there are still large gaps. Species that are eaten, but whose signature is not currently recognized cannot be identified through this method.
Another new method is being developed at Memorial University. It analyzes the slurry in a seal's stomach for DNA evidence of the presence of cod and other prey fish. This method is so new that the first data from it were just presented in late 2007 at conferences in Finland and South Africa. The early results are very exciting, though, and it is expected to give a much more accurate and unbiased definition of the content of a seal's stomach than provided by the study of the hard parts.
Knowing the seal's behaviour and capabilities within its environment are other areas where huge gaps still exist. Stenson's team is using satellite telemetry to determine where the seals go at sea and their diving patterns, both of which will identify where seals feed and what their potential prey can be geographically and at depth. An interesting offshoot of the tagging work is that it has turned the seals into DFO research assistants, as each tag is equipped to collect oceanographic information such as water temperature and salinity. These data are being used by oceanographers to improve their ocean models that can, in addition to other things, improve our search and rescue capabilities.
The tags glued onto a hooded seal will drop off when the fur is moulted, but during that time the seal can be tracked by satellite providing valuable insight into its geographic range and diving capability - both of which shed light on its diet and how it uses its environment to obtain food.(Photo: DFO, Science)
The question of the seal's true impact as a cod predator is not going to be an easy one to resolve, but the scientific work is now well underway. Seals and cod exist in a complex ecosystem that includes numerous other players, and this does not enable easy analysis or simple answers to questions about problems such as the lack of recovery of cod stocks. The whole matter boils down to a fishy take on the ancient parable about the eight blind men and the elephant that showed if you only look at something from one angle you will have a very different understanding than if you look at it from all sides. Of course, in this case, the methods and data sets needed to study the seal-cod equation from all sides are still under construction. But as Stenson says, "We are getting the tools in place that will let us figure out the important variables and allow us to really focus our research. We're definitely on the uptake now."
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