Changing Dynamics of Marine Life in the Gulf

One of the difficulties in fisheries science - at least when it comes to understanding the rise and fall of fish stocks - is coming to grips with the various components of fish productivity such as reproduction, growth and mortality. In the southern Gulf of St. Lawrence most baseline information comes from the 'multi-species bottom trawl survey', an annual survey that has been carried out each September since 1971. The survey provides a profile of the changes in that part of the ecosystem composed of fish and larger invertebrate fauna, reports http://www.megafishnet.com/ with reference to Fisheries and Oceans Canada.

Based largely on data from the annual survey, Fisheries and Oceans Canada (DFO) scientists have concurrently been studying two interrelated aspects of the ecology of southern Gulf of St. Lawrence marine fish. The first is an attempt to describe changes in the long-term species composition in the community and seek explanations for those changes. The second is a more detailed examination of the mortality rates of a number of species such as cod, for which the death rate of adults appears to be severely limiting recovery and in many cases leading to continued decline.

These observations have been the subject of ongoing study by DFO scientists Hugues Benoît, Doug Swain and their colleagues working out of the Gulf Fisheries Centre in Moncton, New Brunswick. Mr. Benoît notes, "In our study of the southern Gulf fish community, our objective was to look at hypotheses for those changes in the relative abundance of over fifty fish species. That is traditionally done by a correlative approach where you look at how abundance of something has changed over time and what factors have a similar or opposite pattern that might explain that change. In our case, we took a traits based approach, looking at the association between fish abundance trends and four species-specific ecological traits: Susceptibility to fishing, susceptibility to predation from seals, thermal preferences of the species based on their bio-geographic distribution, and diet. These traits were chosen to respectively reflect potential species responses to fishing, seal predation, changes in ocean temperatures and changes in prey availability. What we found is that the first three traits, the susceptibility to fishing, to seals, and their thermal preferences, all jointly explained a fairly reasonable proportion of the dynamics in the community."

In the 1970s and early 1980s, the marine fish community of the southern Gulf was composed mainly of larger-bodied species including cod, white hake, American plaice, and larger flatfish. That community dynamic has now shifted to a composition of considerably smaller-bodied species. Specifically it has transformed from one where it was dominated by species that were more susceptible to fishing, more susceptible to seal predation and tended to be warmer water species - to one where they're less susceptible to fishing, less susceptible to seal predation and to more colder water species.

Mr. Benoît: "Part of the explanation we propose for the changes in the community is that the species that were vulnerable to fishing were reduced to very low levels of abundance and many of those were also vulnerable to predation by seals. As the seal population increased, we hypothesized that a lot of the mortality that used to come from fishing, has since come from predation by seals."

But as he notes, the issue of seal diet is a complex one, and it is difficult to say definitively how present day seal consumption of cod and other species fits into the mortality equation. "We have what you might call indirect evidence of an effect of seals. Direct evidence would be to have estimates of how much and which sizes of these species seals eat, thereby allowing us to calculate what contribution of mortality they cause. The diets that are available for seals suggest they don't eat much cod and that those that they do eat are smaller cod; whereas we estimate natural mortality to be the highest on larger cod.

However, there are reasons to suspect that existing data on seal diets in the southern Gulf do not properly represent the mix of species and sizes that the seals actually eat. While seal diet studies continue, we are also looking at different lines of evidence to gauge the relative support for different hypotheses concerning the continued decline in numerous large demersal fish (fish that live on or near the bottom of the sea), despite little fishing."

More detailed work has been done aimed specifically at teasing out the effects of fishing and other causes in the mortality rates of adult fish. With information on relative fish abundance and on growth rates collected during large-scale surveys, scientists can estimate the mortality rates of fish, but attributing the rates of decline - those due to natural causes such as predation versus those due to human factors such as fishing - is a harder nut to crack. A moratorium on directed cod fishing from 1993-1997 gave researchers a unique opportunity to estimate natural mortality. It was at that time that it became clear for the southern Gulf stock, natural mortality was probably twice as high as it was in the 1970s. Since then it has become apparent that mortality unrelated to fishing has continued to rise for cod and also risen in other fish species such as white hake and winter skate. The trends in natural mortality follow increases in grey seal abundance, but that alone is insufficient to conclude definitively that predation is the main cause.

There are other issues that are equally difficult to factor in with any degree of certainty to the community change and mortality rate story, such as changes in ocean temperatures, industrial pollutants, acidification and other changes in the foodweb. On the matter of water temperature alone, the Gulf of St. Lawrence experienced, beginning in the mid '80s and culminating in the mid 1990s, a cooling of the bottom water temperatures. A warming to more average temperatures has subsequently occurred. A shift in the composition of the fish community coincided with the temperature trends. Cooling temperatures were accompanied by a shift toward colder water species that the researchers surmised was probably a combination of some species simply shifting their distributions and moving into the Gulf and others experiencing conditions favorable for recruitment.

The story of the southern Gulf is not one merely of species declines. As some species have declined in abundance, others have thrived. The Southern Gulf community is now composed of smaller-bodied species and researchers think that is due, at least in part, to the fact that there are no longer these large fish that used to eat them. You remove cod, hake and a lot of these larger-bodied species and their former prey can now increase in abundance. There are suggestions, for example, that dramatic decreases in predator abundance has strongly contributed to large increases of shrimp populations.

As Mr. Benoît points out, "Based even on the most recent years of the survey, there continue to be changes that provide additional context that furthers our understanding of the underlying causes of mortality. These studies are ongoing. I would say where we're going specifically is to test some detailed hypotheses for the natural mortality of cod that would apply to other species that show similar patterns in mortality, as well as to refine our understanding of the impact of seals and the upper bounds to the impact they might be having."

The issues are far from resolved which means that these types of studies and surveys will continue. The resulting data will inform the management of fisheries on a year-to-year basis, in addition to improving our understanding of human activities affecting marine fish populations and their ecosystems.