Fish larvae and early postlarvae proved highly vulnerable to the toxins of the dinoflagellate Protoyonyaulax tamarensis. Capelin Mallotus villosus and Atlantic herring Clupea harengus harengus larvae from the St. Lawrence Estuary, Canada, were exposed to variable concentrations of a toxic strain (treatment) and a non-toxic strain (control) of P. tamarensis. When exposed directly to the dinoflagellate, mortality due to the toxin (treatment minus control) was strongly correlated with the percentage of larvae that ingested cells. At cell concentrations (1500 cells ml-') that compared with local bloom densities, mortality reached 92 and 77 % d-' in capelin and herring, respectively. The mortality of herring postlarvae fed toxic microzooplankton ranged from 17 to 36 % d-' Age-and dosedependent variations in vulnerability were linked to early ontogenetic changes in the feeding efficiency and food selectivity of both species. We conclude that the present proliferation of toxic dinoflagellates in coastal waters could jeopardize the early survival of fish and recruitment to fisher~es by narrowing the spatiotemporal window within which spawning leads to successful reproduction.
ABSTRACT. Three adult and three juvenile belugas were fitted with satellite-linked radio tags in eastern Hudson Bay in midAugust 1993, and one adult was tagged in mid-October 1995 in extreme northeastern Hudson Bay. The tags transmitted data on dive behaviour, and the receiving satellite calculated positions by Doppler-shift triangulation. The belugas tagged in summer in eastern Hudson Bay made no directed or long-distance movements while the tags were attached. Their range did not include the Belcher Islands, and belugas observed in aerial surveys of those islands do not appear to belong to the eastern coastal stock. The single beluga tagged in northern Quebec in October moved into the deep water of western Hudson Strait and travelled east along the southern coast of Hudson Strait, slowing up on reaching shallower water off Salluit and near Charles Island. This whale was still off Salluit when the tag stopped transmitting.All the tagged belugas dived intensively while the tags were attached, although there were individual differences, some belugas diving noticeably less than others. Dive behaviour varied over time, with periods lasting several days of concentrated diving interspersed with periods of less intense diving. 'Intense' periods entailed diving for up to 80% of the time. All belugas, even the one that was in deep water in Hudson Strait, showed dive depth characteristics that were consistent with diving usually to the bottom. However, all belugas always-even in deep water-made dives that usually lasted less than 10 min and very seldom lasted more than 12 min. Belugas tagged as pairs of adults and young showed striking correlations of dive behaviour. The data obtained indicate that it would be appropriate to correct aerial surveys by adding 85% to aerial counts.Key words: beluga, Delphinapterus leucas, dive behaviour, migration, movement, Hudson Bay, Hudson Strait, visibility, dive speed RÉSUMÉ. Mi-août 1993, on a équipé trois bélougas adultes et trois bélougas juvéniles de radio-émetteurs en liaison avec un satellite dans l'est de la baie d'Hudson et mi-octobre 1995 on a équipé un adulte aux confins nord-est de la baie d'Hudson. Les émetteurs ont transmis des données sur le comportement en plongée, et le satellite récepteur a calculé les positions par triangulation de décalage Doppler. Les bélougas équipés en été dans l'est de la baie d'Hudson ne se sont pas déplacés vers un endroit particulier ou sur une longue distance pendant que les émetteurs étaient fixés. Leur territoire ne comprenait pas les îles Belcher, et les bélougas observés lors de survols aériens effectués sur ces îles ne semblent pas appartenir au stock côtier oriental. Le seul bélouga équipé en octobre dans le nord du Québec s'est rendu dans les eaux profondes du détroit d'Hudson occidental dont il a longé la côte méridionale vers l'est, ralentissant au niveau des eaux moins profondes au large de Salluit et près de l'île Charles. Cette baleine était toujours au large de Salluit quand l'émetteur a cessé de fonctionner. Tous les bélougas é...
The environmental cues triggering larval capelin (Mallotus villosus) emergence in the St. Lawrence estuary and in coastal Newfoundland are different. In the estuary, emergence from the intertidal and subtidal spawning grounds starts with dusk and ends with dawn, indicating an active response to low light intensity. In the laboratory, emergence from undisturbed sediments collected in the field is perfectly synchronized with the dark phase of the illumination cycle. Nocturnal emergence would represent an adaptation reducing vulnerability to visual predators. Previous work has shown that in Newfoundland, capelin emergence from intertidal beaches is triggered by abrupt changes in water temperature following the sporadic advection to the coast of surface water masses characterized by low predator and high prey abundances. We argue that regional differences in the emergence pattern of the species represent different strategies to avoid predation at the onset of the planktonic drift when the vulnerable yolk sac larvae are densely aggregated. Selection acting on the behaviour of the early larval stages could contribute to stock differentiation in capelin.
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