A Large-Volume Pump System for Studies of the Vertical Distribution of Fish Larvae Under Open Sea Conditions

Abstract

A large-volume pump system (2.8 m3 min-1) for sampling fish larvae under open-sea conditions is described. Comparative efficiency trials by day and night showed that the pump was generally as efficient, or in some cases more efficient, in capturing larvae than vertically hauled 200 μm WP2 nets, though there was some evidence of visual avoidance by particular larval size classes during daylight. The pump system is particularly appropriate for investigating fine-scale vertical aggregations (1–10 m3) of larval fish in relation to the distribution of their food organisms.INTRODUCTIONStudies of the distribution of larval fish and their food organisms in relation to physical structure in the water column require sampling techniques capable of resolving fine-scale temporal and spatial distributions. As an alternative to conventional nets, large-volume pumps, sampling at rates in excess of 1 m3 min-1; provide such a capability. Major benefits of using large pumps in addition to temporal and spatial resolution are that a wide range of sizes of plankton including larval fish can be sampled simultaneously in relation to physical and chemical properties of the water column; there is reliable control of the volume of sample filtered and problems of clogging of towed nets are avoided; long series of sequential samples can be taken in studies of small-scale distribution; and instrumentation with in situ CTD and fluorometers at the intake enables real-time control of sampling in relation to physical structure.General engineering considerations for using such pumps have been reviewed in detail by Miller & Judkins (1981), and a particular area of application has been in power-plant entrainment studies in shallow fresh water (Portner & Rhode, 1977; Bowles & Merriner, 1978; Gale & Mohr, 1978; Ney & Schumacher, 1978; Elder et al. 1979; Leithiser, Ehrlich & Thum, 1979; Cada & Loar, 1982).