What's Holding Back the Cod Recovery?
The early 1990's saw a historic collapse of Canada's Atlantic cod fisheries. At the time, most people looked forward to an early recovery. Instead, most fisheries have stayed closed. Cod populations remain depleted, and the reasons remain disputed. In the mean time, there is a continued fishing on some of the cod stocks that may explain the delay in specific cod population recovery.
Standard theory suggests that when fish stocks get thinned out, that means more food on average for each remaining fish, and thus, faster growth. But in the northern Gulf of St. Lawrence, individual cod have continued to look skinny and emaciated. Research scientist Jean-Denis Dutil of the Maurice Lamontagne Institute (MLI) in Mont-Joli, Quebec, began to ask why. His findings about the poor condition of individual fish relate to the continued weakness of population numbers in general.
“I had been working in aquaculture, with good-looking, healthy fish,” Dr. Dutil says. “The wild cod looked very different. We started investigating how that would affect their ability to cope.”
Dr. Dutil and colleagues started monitoring cod in land-based tanks, limiting their food until their condition resembled that of the emaciated wild cod. In a striking result, the researchers found that wild fish in that condition would have trouble surviving.
Scientists often gauge fish fitness by measuring what is known as the condition factor (CF), derived from the weight to length ratio. For a healthy cod, the CF works out to about 1, and for a cod in top shape, around 1.1 or 1.2. CF's for northern Gulf cod came well down the scale, often .6 or .7.
“Observations showed us that fish in that condition can't swim as fast, to catch prey or get away from predators,” Dr. Dutil says. “Their resistance to disease and their chances of successful spawning diminish.”
In the wild, cod take on weight in the summer. They feed less in winter, and need to use the energy that was stored in their liver and muscle over summer. As spring approaches, much of the remaining stored energy goes into the gonads for spawning, leaving the cod weaker. A fish in good shape can cope with all that; one in poor shape will have problems.
“We realized that many northern Gulf cod were starving to death and, in their weakened condition, were more vulnerable to disease and predation,” Dr. Dutil says.
So poor condition was a major factor leading to poor growth, survival, and reproduction. But what was causing the poor condition? There the situation gets less measurable.
Decades back, Gulf of St. Lawrence cod were in better bodily condition. In the 1980's and early 1990's, their average condition worsened, and stayed poor after the great decline of cod stocks. Although excessive fishing appears to have caused most of that drop in abundance, environmental factors were also at work, in ways still poorly understood.
When water temperatures in many Atlantic areas cooled in the late 1980's and early 1990's, that was part of a wider change in the ecosystem. Cod live within a narrow range of temperatures. If their habitual waters get too warm or cold, they can migrate till they find a comfortable zone. But will they find, for example, the right levels of oxygen? And, if broad environmental changes are taking place, what's happening to their food?
“Cod prey on more than 200 different species,” Dr. Dutil says. “We have too little information on those food supplies and what's been happening with them. But in the northern Gulf, it seems obvious that a major cause of emaciation and poor condition is changes in the food supply.”
Even though there has been some improvements observed in the northern Gulf cod stock, some scientists have put forward a different hypothesis for the general poor physical condition of Gulf cod. They have speculated that high fishing pressure over a long period weeded out bigger and stronger fish, leaving a weaker gene pool with fish less fitted for survival.
To test one aspect of that hypothesis, Dr. Dutil and colleagues put Gulf of St. Lawrence cod and Bay of Fundy cod in different tanks at MLI, and grew them in optimal conditions. With unlimited food, the Gulf cod did better than the Bay of Fundy cod in cold water, suggesting that their problem was less genetic than environmental.
Dr. Dutil explains that the environments of Gulf cod, Grand Banks cod, Newfound-land's northern cod, and indeed all the once-plentiful cod stocks north of Halifax, were never ideal to start with. By comparison, individual cod in waters of the Faroe Islands, Iceland, southwest Nova Scotia, and New England all have better condition factors. Their superior environments, associated with higher bottom temperature, let them grow faster, in stocks that are naturally more productive.
When one recalls the legendary abundance of Canada's cod before the collapse, the idea that many of our waters have poorer food or other conditions for growth might seem to defy common sense. But, poor individual production in these earlier cod stocks was offset by large population numbers living over a very broad area. The huge and diverse population structure included a strong component of larger, older fish. Older females, which spawn at different times than younger ones, produced better eggs and increased the reproductive success of the stock.
Even when those cod populations were strongest, however, individual cod grew more slowly than elsewhere. The northern Gulf of St. Lawrence, in particular, offered a colder environment with less oxygen. For cod stocks there and in most other Canadian areas, the natural intrinsic productivity was low, and in the late 1980's and early 1990's, it got worse.
In short, the former great abundance of Gulf and Newfoundland cod rested on a shaky foundation. They were more vulnerable than thought, and the continued onslaught of high-pressure fishing after the Second World War, together with untoward changes in the ocean, did them in.
Regrowth may take a long time. Even then, Dr. Dutil says, “We will need to be more respectful of the challenges the species has to face.” Management approaches will need to change. In future, government and the fishing industry will need to take better care of larger, older fish, and the stocks in general.
In this aspect of their varied lab and field work, Dr. Dutil and colleagues have produced a striking picture of a major obstacle to cod recovery. If poor condition caused mortalities in northern Gulf cod, it could do the same elsewhere. Other potential factors holding back cod recovery are many, including fishing pressure and competition with and predation by seals. But by highlighting the condition of individual fish and their chances of survival in a changed environment, Jean-Denis Dutil has enriched the debate and helped point the way to better conservation.