Emerging science workshop challenges fish paradigms

by Sean Sullivan NAMA Marketing, Development and Outreach Associate


Everyone knows that salmon are born in rivers, spend their adult lives in the ocean and return to the very same rivers in which they were born to spawn. Until recently most people thought that anadromous Salmon were exceptional among fishes in their loyalty to spawning grounds. It is commonly thought that most marine fish were broadcast spawners and did not exhibit natal homing (returning to spawn where they were born). It was assumed that spawning areas attracted aggregations of a genetically diverse group of fish from regional stocks that found the areas suitable.

It turns out that many if not most fish that live their whole life in the sea are more like salmon than we ever thought. In fact, we are just learning that many fish return to the location where they were born (or nearby that location) to spawn. Cod, for example, return to spawn year after year in the exact same piece of desirable real estate where they were born. It may be a notable bank or simply a small pebbly plateau that is somehow just the ‘right spot.’  This is just one of bits the emerging scientific knowledge that was reported recently at a workshop on Reconciling Spatial Scales and Stock Structures for Fisheries Science and Management.

The implications of this emerging scientific consensus are fairly drastic if you are a fisherman, fisheries manager, fisheries scientist, or, as in the case of us at NAMA, advocates for community based fishermen.  Not surprisingly, fishermen whose scale of operation best matches the scales of fish distributions in the ecosystems where they choose to fish are more likely to be successful stewards of their ecosystems and their fisheries more diverse and sustainable over the long haul. The conference produced a slew of other earth shaking ideas and notions, but before we get to those, lets walk down the path of understanding the example above --our beloved codfish.

One of the great mysteries scientists, fishermen and others have been struggling to understand is how in some areas such as in the Western Gulf of Maine, overfished cod have rebounded, while in the Eastern Gulf of Maine and Georges Bank, they are still struggling or absent. It turns out part of the problem is that it has always been thought that breeding fish would over time re-stock the ocean just through the magic of winds and currents moving those very young cod around and mixed populations converging on suitable spawning sites. So if we weren’t managing with cod’s homing tendencies in mind we were missing the boat – pun sort of intended.

But if cod are like salmon, and humans put up a barrier - in the case of salmon say a dam, in the case of cod say a net - that prevents them from returning to their natal spawning grounds, an entire genetic population of fish may disappear. A river or other spawning habitat may be repaired and regenerate, but it is thought that the timeframes for nature to re-stock a river may be in the hundreds if not thousands of years. If a cod breeding ground, say in the Eastern Gulf of Maine, is wiped out by overfishing or habitat changes, there are no more fish that have that specific genetic code linked to them returning to that specific location. A distinct fish population suffering such a fate is GONE for good.

Cod population crashes are well documented in many North Atlantic fisheries, as in the North Sea and of course here in Northwest Atlantic region, including Canadian waters, Georges and Grand Banks, and of course in the Gulf of Maine. A recent sentinel fishery (a fishery designed specifically to gauge fish populations) in the Eastern Gulf of Maine conducted in conjunction with our friends at the Penobscot East Resource Center, has shown that there are more halibut in the Eastern Gulf of Maine than there are cod, and even the numbers of halibut are extremely low. And with little to no fishing pressure these formerly abundant fish are still not returning.

Why they are not returning could in fact be the result of fishing pressure and/or environmental changes resulting in the disappearance of a sub-stock (or distinct population).  In addition to overfishing the population in question and/or overfishing their food fish, toxic pollution, climate change, excess fertilization and a variety of changes in the food web can lead to collapses in fish populations.  So somehow the breeding cod for that area were wiped out and couldn’t replenish themselves; and because of that, there are no adult cod in those areas. Quite literally, the Eastern Gulf of Maine used to be one of the most productive fishing areas, equal to Stellwagen Bank, Georges Bank and other well known historical fisheries. Today there is no commercial fishery for cod there at all.

Complicated interdependencies between species may also result in dramatic changes in fisheries.  Again in this case we will use cod as an example, and that species dependence on specific prey. For example, cod in parts of Canada fed primarily on capelin, a small fish related to herring. The capelin provided the nutritional basis for cod to spawn successfully. If there are no capelin, cod will feed on shrimp. However, if they are feeding on shrimp, they are less successful and in some cases will not spawn at all.  A similar disappearance of river herring from cod spawning grounds in Eastern Gulf of Maine may similarly be linked to the disappearance or non-recovery of sub-populations of cod there.  And we should not forget that fishermen are also interwoven into these interdependencies.  We cannot forget the human element in the marine ecosystem.

I am not a scientist, and I am sure there are nuances I may not be getting right. But, the example of cod and other species returning to their natal spawning grounds is a "spatial" issue as well a biological one. The behavior argues that the animals are related more closely to a particular spot in the ocean than was previously thought. Fisheries managers largely manage fish as though they are a single uniform stock that doesn’t have strong preferences for where they live, breed or eat as long as fundamental needs are met. We now know that is not true. Management will have to change to acknowledge and accommodate this "spatial" relationship.

These emerging scientific ideas are forcing a new understanding that requires fisheries managers to expand their age old practice of looking at the ocean only temporally (fluctuations over time) to include spatial needs in a much more detailed way if they want to be successful at managing the ocean. While it was acknowledged at this workshop that spatial considerations would be key in the transition from single species management to ecosystem based fisheries management the path to get there was not laid out and is clearly the next step that must be taken.