Everyone knows that Salmon are born in rivers, spend their adult life in the ocean and return to the very same river they were born in to spawn. Until recently most people thought that Salmon were the exception among fishes. It was commonly thought that most other fish were dispersal spawners and did not exhibit natal homing. In other words the eggs of the fish were released into the ocean's currents, the fish eventually grew and settled somewhere among a school, then spawned with that school.
It turns out that most fish are more like salmon than we ever thought. In fact, most fish return to the location they were born (or nearby that location) to spawn. Cod for example may return to the exact same hump of rock where they were born to spawn year after year. This is just one of the emerging scientific trends that was reported recently at a Reconciling Spatial Scales and Stock Structures for Fisheries Science and Management Conference.
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 small scale community based fishermen. 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 just the example above, our beloved codfish.
One of the great mysteries scientists, fishermen and others have been struggling to understand is how some populations of over-fished cod have rebounded, such as in the Western Gulf of Maine, while those in the Eastern Gulf of Maine and George's Bank are still struggling. It has always been thought that a dispersal breeding fish would necessarily over time re-stock the ocean just through the magic of winds and currents moving those very young cod around.
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 population or stock of fish may disappear. A river may 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 is wiped out by overfishing, the fish that have that genetic code that tells them to return to say the Eastern Gulf of Maine to spawn are gone. Let's repeat that, those fish are GONE.
Cod population crashes are well documented most spectacularly in Canada, but have also happened in the North Sea and here in the Eastern Gulf of Maine. A recent sentinel fishery (a fishery designed to gauge fish populations) in the Eastern Gulf of Maine conducted in conjunction with our friends at the Penobscot East Resource Center, showed that there are more Halibut in the Eastern Gulf of Maine than there are cod (and the numbers of Halibut are still 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 resulting in an "extirpation" of a stock. So, in other words somehow the breeding fish for that area were wiped out and because of that, there are no adult fish in those areas. Quite literally, the Eastern Gulf of Maine used to be one of the most productive fishing areas, equal to Stellwagen Bank, George's Bank and other well known historical fisheries. Today there is no commercial fishery there at all.
To make things even more complicated, additional research presented at the conference shows an inter dependency between some species, again we will use cod as an example, and their main 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.
I am not a scientist, and I am sure there are nuances I may not be getting right. But, the first example of Cod returning to their natal grounds is a "spatial" relationship. This connection 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. We now know that is not true. Management will have to change to acknowledge this "spatial" relationship.
The example of the Cod being interdependent on Capelin to successfully breed is an example of an "Eco-system" dependency. In the past fisheries managers have managed fish stocks as if there are no eco-system dependencies. Specifically, managers are by law required to manage commercial species to attain a maximum sustainable catch of all species. However species do not exist in a vacuum, they exist in an eco-system where fish eat other fish.
Now these emerging scientific ideas are forcing an new understanding that not only must we look at the ocean spatially, but temporaly (fluctuations over time) in a much more detailed way if we want to be successful at managing the ocean.