Capelin Mallotus villosus (Pisces: Osmeridae), glaciations, and speciation: a nomothetic approach to fisheries ecology and reproductive biology

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Abstract
Capelin Mallotus villosus in the North Pacific and Northwest Atlantic are characterized by a wider latidutinal distribution (cool temperate to subarctic), lower number of vertebrae (64.45 to 68.50), lower biomass, lower fecundity and larger eggs than are capelin in the Central-Northeast Atlantic (exclusively subarctic, 69.16 to 71.54 vertebrae). Capelin in the North Pacific and Northwest Atlantic spawn intertidally over a wider range of latitudes (42 to 72.degree. N), temperatures (1 to 14.degree. C), salinities (from brackish to oceanic water), and at a higher mean temperature (7.5.degree. C), than do populations in the Central and Northeast Atlantic (offshore spawning, 63 to 76.degree. N, 0.94 to 7.degree. C, 32.0.permill. to 34.6.permill., 4.degree. C respectively). These differences indicate that capelin are towards the r-endpoint of the r-k continuum of life history strategies in the North Pacific-Northwest Atlantic and towards the k-endpoint in the Northeast Atlantic. A nomothetic explanation is proposed. During the peak of the last glaciation, the major present-day spawning grounds on the Pacific rim were ice-free. Capelin experienced little change in their accustomed environmental conditions and spawned intertidally over a wide geographic range. In contrast, North Atlantic spawning grounds were ice-covered. Accordingly, offshore spawning habits evolved. Intertidal spawning in the North Atlantic probably did occur between 40 to 55.degree. N along the European coast and in the Mediterranean Sea where ice did not reach the sea. Capelin populations underwent k-selection, and stenothermy, stenohalinity, low fecundity, large egg size and high biomass evolved in populations of deep-spawning capelin of the North Atlantic in response to the greater environmental predictability in these offshore habitats. During the Holocene hypsithermal, capelin from the Pacific Ocean (intertidal spawning) repopulated Canadian Arctic waters and Northwest Atlantic. Deep-spawning capelin continued to reproduce in the North Atlantic where bathymetry, spawning temperatures and salinities were optimum (Grand Banks, Iceland, Barents Sea). Minor stocks of beach-spawning capelin which now inhabit some Norwegian fjords may have been derived from the Northwest Atlantic, via the North Atlantic Drift, and/or from relict populations which spawned intertidally along the ice-free Western European/Mediterranean coast. The implications of this hypothesis for the taxonomic status of capelin are discussed.