On Tuesday, October 9th, Kenneth Frank, from the Department of Fisheries and Oceans at Bedford Institute of Oceanography in Nova Scotia, delivered a lecture entitled “Cascading Effects of Removing Top Predators from Marine Ecosystems”.  Dr. Frank’s recent work has focused on the concept of cascades in marine ecosystems, and this lecture was a general synopsis of some of his findings and suppositions.

The natural state of marine ecosystems is bottom-up, roughly meaning that the lower levels of the food chain are essentially the building blocks of the system.  A bottom-up ecosystem implies that the exploitation of top predators would have little or no impact on the lower trophic levels.  Under this commonly exhibited pattern, positive correlation is observed between the trophic levels in the food chain.  That is, an increase in phytoplankton begets an increase in zooplankton which in turn begets an increase in foraging species and so on through demersal and pelagic predators; likewise would be the case for decreases.  However, this typical and expected correlation has been observed to not hold true in certain areas where cod (a top predator) has been heavily exploited.  For illustrative purposes, these observations showed that exploitation of top predators created an increase in foraging species which created a decrease in zooplankton which, in turn, created an increase in phytoplankton.  In short, a top-down control pattern had manifested - the ecosystem exhibited negative correlations between adjacent trophic levels in the food chain.  Top-down control implies that exploitation in top predators could have serious rippling effects in a marine ecosystem.

It was originally opined that relatively large and complex ecosystems would be immune to top-down control, but the eastern Scotian Shelf offers a clear exception.  With the depletion of cod, foraging species as well as snow crab and northern shrimp dramatically increased population size.  Zooplankton decreased, phytoplankton increased and nutrients steadily dropped.  The commercial value of lobster, snow crab and northern shrimp grew from 8% to 85% of the value of the fishery.  While this might bode well for certain members of the human fishing community, the efforts to revitalize the cod population through the enforced moratorium of 1993 have not seen success.  Why?  The disproportionate dominance of the foraging species is the likely answer - the young and eggs of the predators are often the prey for these other, now pervasive, species.

Reversals of trophic control from bottom-up to top-down are not desirable if one wishes to see the population recovery of an exploited top predator such as cod.  But how common is this trophic control reversal and under what conditions is it likely?  Dr. Frank analyzed 39 exploited fish stocks, 17 of which displayed top-down control patterns.  Differences in exploitation levels did not adequately account for the top-down pattern, but, interestingly, water temperature did.  All cases exhibiting top-down patterns took place in colder water temperatures (less than 5 degrees Celsius) which influences such factors as growth rates and age at maturity.  The other significant indicator was lack of species diversity.  Conversely, warmer water and species rich systems appeared more resistant to higher levels of exploitation without transforming to a top-down pattern.

In summary, Dr. Frank reiterated the importance of understanding trophic cascades since fishing effects are not easily reversed, if at all, once top-down transformations have occurred.  That is, targeted fish species are more at risk in colder water systems with relatively less species diversity; large marine systems are not immune.  But, as one student asked, the question of today is will cod recover? Dr. Frank does not seem optimistic, though he clearly did not want to speculate.  The short answer is possibly, though not likely.  It has been 15 years since exploitation has been greatly reduced (or eliminated), and not much recovery has been observed.  This leads one to believe that the eastern Scotian Shelf may be in a permanently altered state that may not be able to recover on its own.  Should we, then, intentionally deplete the foraging species to allow a cod recovery?  Given that we clearly have much to learn about eco-system dynamics and given our track record of creating more problems with our solutions, I personally remain skeptical of such a measure and may even characterize it as a cat in the hat approach to problem solving.