Many global marine fisheries have collapsed, or are at record low levels of abundance. Continuing exploitation and the uncertain impacts of climate change are adding further pressures on fish resources. New strategies are therefore required to assist in our management and conservation efforts. One such development will be to incorporate the extent and dynamics of spatially-associated biological differences that exist among fish stocks into stock assessment. Such information is important since most exploited fish species comprise assemblages of individuals that differ in their vital rates of growth, reproduction, migratory tendency and mortality. It therefore becomes desirable, for example, to match the level of fishing intensity to the projected rate of replenishment following harvesting, thereby reducing the probability that individual stocks will become extinct. Conserving such biological differences among stocks is also important in the maintenance of genetic diversity in wild fish populations, so endowing them with greater evolutionary potential for adapting to changes in the environment. A critical component of such work involves the testing of hypotheses relating to the relative contributions of such factors as dispersal of eggs and larvae, and behaviour of juveniles and adults in maintaining such biological differences. By integrating research efforts across each of the three major UK fisheries agencies (FRS, CEFAS, AFBI) with partners in UK Universities, a NERC Institute and an international cod expert from Denmark, we will for the first time examine the extent, patterns and stability of cod (Gadus morhua) population structuring throughout UK waters. We will employ existing (microsatellites) and new (single nucleotide polymorphisms) genetic markers to assess the extent to which individuals from different spawning groups interbreed, and will compare these indirect methods of dispersal with direct measure of fish movement taken from chemical analysis of ear-bones ('otoliths') and individuals that have been electronically 'tagged'. We will then input these data to tailored cod population models that will test the likely effects of young-stage and adult dispersal on observed patterns of stock separation, as well as simulating the likely consequences of spatially-based differences in stocks on such things as fishing effort, stock recovery and the design of marine protected areas. The proposal will represent the first opportunity for UK Universities and all three UK Fisheries agencies to work together on a problem that has to take account of the interdependence of fish stocks across large regions of UK and adjoining waters. The proposal will generate new genetic estimates of stock separation, especially in areas of uncertainty such as the Celtic and Irish Seas and the Southern North Sea and English Channel, as well as the provision of new theoretical tools (population models) than can be used to forecast the impact of continued fishing pressure and environmental change on cod populations. Moreover, the tools will provide information on the appropriate spatial scale and distribution of marine protected areas and the probable rates of stock recovery in a species that is now formally endangered and listed in the IUCN Red List (http://www.iucnredlist.org/).
Conserving genetic diversity in animal populations is important for sustaining their ability to respond to environmental change. However, the 'between population' component of genetic diversity (biocomplexity) is threatened in many exploited populations, particularly marine fish, where harvest management regions may be larger than the spatial extent of genetically distinct sub-populations. Using single nucleotide polymorphism (SNP) data we delineated the geographic limits of three population units of Atlantic cod (Gadus morhua) in northwest European waters. Two of the populations co-habit the North Sea, and trawl survey data showed differing trends in their abundances. We developed a spatial model of these units to simulate population dynamics under spatial patterns of harvesting, Competition between units during the pelagic juvenile stages in the model led to suppression of the more localised northern North Sea (Viking) unit by the more widespread (Dogger) unit, and its premature extinction under some spatial patterns of fishing. Fishery catch limits for cod are set at the scale of the whole North Sea without regard to such sub-population dynamics. Our model offers a method to quantify adjustments to regional fishing mortality rates to strike a balance between maximising sustainable yield and conserving vulnerable populations.