The nominal catch of blue sharks (Prionace glauca) reported for the Canadian Atlantic grossly underestimates the annual catch mortality of about 1000 tonnes (t), making blue sharks the most frequently caught large shark in Canadian waters. Although blue sharks accounted for 99% of all sharks landed at recreational shark fishing tournaments, tournament catches accounted for only 3% of total fishing mortality. Standardized catch rate indices suggested a decline in blue shark abundance of about 5%–6%·year–1 since 1995. An increased mortality rate in recent years was suggested by a decline in the median size of blue sharks in the commercial catch. Two independent calculations suggest that North Atlantic catches exceeded 100 000 t, with catch mortalities ranging between 26 000 and 37 000 t. Because tagging studies indicated that blue sharks are highly migratory with a single population in the North Atlantic, the Canadian contribution to overall population mortality accounts for only 2% of the total. The fact that blue shark populations are relatively productive and resilient may help explain their persistence in the face of high international catch mortality and a decline in relative abundance.

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The neurotoxin domoic acid is produced in quantity by the diatom Pseudo-nitzschia multiseries and is released to the environment directly and indirectly via food chains. Presumably there is a mechanism for the biodegradation and disposal of domoic acid and as bacteria are logical candidates for such an activity, a search for bacteria competent to carry out biodegradation of domoic acid was initiated. Extensive trials with a wide variety of bacteria isolated mainly from muds and waters taken from the marine environment showed that the ability to grow on or degrade domoic acid was rare; in fact, domoic acid was inhibitory to resting cells or growing cultures of most of these bacteria. In contrast, using enrichment techniques, it was possible to isolate from molluscan species that eliminate domoic acid readily, i.e., blue mussels (Mytilus edulis) and soft-shell clams (Mya arenaria), bacteria that exhibited growth with and biodegradation of domoic acid when supplemented with low concentrations of growth factors. The species that retain domoic acid for lengthy periods, such as sea scallops (Placopecten magellanicus) and red mussels (Modiolus modiolus), only occasionally yielded bacteria with this capability. The differences may be a result of the mechanisms used by the different shellfish in dealing with domoic acid, i.e., freely available in the blue mussels and soft-shell clams but likely sequestered in the digestive glands of sea scallops and red mussels and thus, largely unavailable for bacterial utilization. The results show that Mytilus edulis and Mya arenaria, almost uniquely, are prime and reliable sources of domoic acid utilizing bacteria. These findings suggest a strong possibility that autochthonous bacteria may be significant factors in the elimination of the neurotoxin in these two species of shellfish.