WHY DO JUVENILE FOURHORN SCULPIN, TRIGLOPSIS-QUADRICORNIS, APPEAR IN THE PELAGIC HABITAT AT NIGHT

Challenging the generally accepted view of the fourhorn sculpin Triglo psis quadricornis as a cold-stenothermal and true benthic species, nov el findings of juveniles in the pelagic habitat of Lake Vattern, Swede n, have been examined and critically evaluated. Young-of-the-year and small-sized, sexually mature individuals, ranging 27-45 and 82-110 mm in total length, were recorded in nocturnal trawl catches from pelagic midwater in August. The sculpin had fed extensively on both benthic a nd semi-pelagic crustaceans, including large cyclopoid copepods, Mysis relicta, Pallasea quadrispinosa, Monoporeia affinis, Gammaracanthus l acustris, and Saduria entomon. Based on an extensive literature review a series of ecological and evolutionary reasons for a pelagic behavio r of this profundal coldwater fish is suggested. Nocturnal, semi-pelag ic feeding, and consecutive, thermotactic, vertical migrations up in m idwater as shown by Wurtsbaugh & Neverman (1988) in order to maximize energy intake after feeding, are possible adaptations of these juvenil e sculpin for minimizing size-dependent mortality due to intra- and in terspecific predation and competition from other deepwater fishes.