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Fisheries management deals with populations or "stocks", usually in reference to geography. TACs and estimates of biomass, for example, normally involve a species and a region, like 2J3KL cod. The effectiveness of management depends partly on how well these divisions reflect reality in the lives and behaviour of fish. The fish in different stocks may differ in their size and growth rate, in the range they occupy, in their habits of migration and spawning, or in a variety of other features of physiology or behaviour. If managers are unaware of these differences, particularly those related to the boundary between adjacent stocks, we may fish one stock too heavily and take less from another than its growth would support.
The same is true of sub-populations within stocks. "Northern Cod", for example, is a management concept which reflects what was known about cod in this region when the first fishing quotas were imposed. Some fishers and scientists have long believed that the northern cod zone - from Hopedale to Cape St. Mary's - contains a number of sub-stocks oriented to particular feeding and spawning grounds along the coast or offshore. Funds required to fully assess this have not been available, and northern cod is still managed as if it were a single stock.
Increasingly, science is recognizing that we must pay attention to the genetic makeup of fish stocks as well. Each individual of any species carries what amounts to a set of complicated biological instructions in every cell of its body. These are carried in the cell nucleus, in "genes". They govern not only how each cell functions, but how the animal itself develops from a fertilized egg to a mature individual. The cells of most animals have thousands of genes, half of them inherited from each parent. This makes for an enormous number of possible combinations, with no two individuals alike.
However, related individuals share many of the same genes, and commonly lack some of the genes shared by another population of the same species. Because of patterns like this, scientists can use genetic analysis to distinguish one stock of fish from another, to estimate the frequency of interbreeding between neighbouring stocks, and to find clues to the ancestry of different stocks.
Any population includes some diversity in its collective gene pool. Without this, a population is vulnerable to disease or changes in its environment. A change in water temperature, salinity or oxygen content, for example, or a decline in the abundance of some preferred food, could mean that fewer young cod survive their first few months, or that fewer adults put on weight and spawn successfully. The greater the genetic diversity in the population, the more likely it is that some will be equipped to cope with such changes. In this regard, diversity is a form of insurance.
It is becoming clear that managing fish stocks requires that we sustain not only the biomass of a population but its genetic diversity as well. The existence of small sub-stocks within a stock like northern cod may be important out of all proportion to the numbers of fish involved. These sub-stocks may well contain valuable genetic traits which are much less common in the rest of the population. For example, studies of the ability of young cod to produce antifreeze show a striking difference between juveniles caught off the Great Northern Peninsula and those caught in more southerly waters. The collapse of the northern cod stock has been most extreme in the northern part of its range. Its ability to recover may partly depend on the existence of exceptionally cold-hardy strains of young cod.
Tagging has long been a standard way of studying fish movements and finding the boundaries and the extent of overlap between adjacent stocks. Typically, scientists and technicians capture fish, hold them briefly in tanks, make weight and length measurements, attach numbered tags and release the fish. When fishers later catch them and return the tags, with details of the time and place of capture, they contribute to what has become a huge database of tagging information.
The Northern Cod Science Program funded a compilation of nearly 40 years of tagging data for cod stocks around the province. The work organized the details of the tagging of 205,422 cod released in 177 different tagging studies since 1954. The database includes up to 47 particular details for each fish at the time of tagging - including things like the time and place of capture, the length of the fish and the depth at which it was caught - and up to 30 similar details for each fish recaptured. Much of this work was done before the use of computers became routine in fisheries science. As a result, the format and the level of detail in the tag data varied greatly. The data had to be re-worked into a consistent format to allow detailed statistical analysis. This work is now complete and culminated in a detailed technical report entitled "The 1954 - 1993 Newfoundland cod-tagging database: statistical summaries and spatial-temporal distributions".
A few highlights that emerge from the database so far:
With four decades of detailed tagging data now assembled in a consistent format, scientists will find it easier to manipulate this information in search of answers to particular questions. Some of this work has begun. One study, for example, used an analysis of tagging data to confirm that fishing mortality on inshore components of the northern cod stock has been very high since the 1950's. This study concluded that it was critical to identify and define these inshore stocks and find ways to conserve cod.
NCSP, in partnership with the Ocean Production Enhancement Network, or OPEN, supported genetics research which revealed major differences in the ability of cod larvae to survive the transition between living off their yolk sac, when they first hatch, and finding their own food after the yolk is depleted. The offspring of some spawners were much better at coping with this transition than others. This suggests that various challenges in the lives of fish, especially early in the life cycle, may "weed out" a higher proportion of those less able to cope. In this way, changes in the environment or the availability of food could prompt changes in the genetic makeup of a population.
More recently, researchers combined the tools of gene probe research and antifreeze analysis to confirm that cod wintering in Trinity Bay are genetically distinct from northern cod which winter offshore. The two populations mingle when the offshore fish migrate to the coast in summer, but analysis of their antifreeze levels could tell them apart.
This work took advantage of the fact that adult cod produce antifreeze much more slowly and at lower temperatures than juveniles. Because of this, the level of antifreeze in the blood of adult fish caught in the spring can help distinguish between those which have overwintered and spawned offshore, in relatively warm water, and those which have spent the winter in much colder water near the coast.
Once the fish in the sample were divided in this fashion, scientists tried several techniques of genetic analysis, to see which was more sensitive in revealing the difference. They could also assess the "genetic distance" between the two populations: that is, how much the inshore fish were genetically similar to each other and different from those which overwinter offshore. The evidence suggests that there is limited genetic mixing. Despite the mingling of these two populations inshore every summer, they appear to separate again in late fall and do not normally interbreed.
This work demonstrates that the "northern cod stock" is not just one stock is not just one stock. During a period of stock rebuilding, the inshore and offshore fish populations may, in fact, make different contributions to the recovery of the "stock" as a whole.
The insights offered by recent genetic research give an added and unexpected value to the huge collection of fish scales and otoliths collected over nearly half a century of fisheries research in the Newfoundland Region. Technicians on research trawlers routinely remove scales and otoliths from a sample of fish in each tow, because these reveal the fish's age. Comparing the age to the length and weight of fish provides a way to monitor their rate of growth. Scraps of dry tissue cling to many of the otoliths collected and stored over the years, and the cells in this tissue still contain recognizable genes.
Hundreds of thousands of such samples, dating back to 1946, represent a huge store of genetic information. Analyses of some of this material are now being used to reveal changes in the composition of different stocks around the province, and to see which of the spawning groups may have contributed most to the recovery from earlier stock declines.
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