Maintaining population diversity and avoiding local depletion is especially challenging in

species where the scale of movement is considerably less than the managed region. This is

illustrated in a study of two North Sea species; Atlantic cod and sandeel. In both, more than

one population inhabits the North Sea, and there are differences in exploitation rate and

environmental exposure. Differences in growth and maturation schedules among populations

make some more vulnerable to fishing pressure. For sandeel, the risk of regional overfishing

was reduced by ICES changing the assessment areas to reflect the scale of life stage

connectivity and local productivity. In cod, the least productive population remains far from

shore and consequently exploitation was low and abundance was maintained. In contrast

some spawning grounds were extirpated in the highly exploited coastal populations and

maturation schedules changed. While cod started to recover the rate differed among

populations. Aggregative behaviour and fidelity in both species also makes them vulnerable

to localised depletion. Cod spawning aggregations were targeted by fisheries due to increased

catchability but the introduction of seasonal closures appears to have reduced this problem.

In sandeel, within season movements and a preference for certain habitat type appears to

maintain high fishery CPUE leading to the potential for bank scale depletion so area closures

have been used to restrict fishing on less productive banks. While the measures applied seem

to have had some benefit, there is a need for a more formal framework to develop and

evaluate spatial management approaches.