Population biology of the deep-water prawn ( Pandalus borealis ) in Balsfjord, northern Norway: I. Abundance, mortality, and growth, 1979–1983

Abstract

The population density, age composition, and ovigerous stock of Pandalus borealis (Krøyer) were studied in Balsfjord (ca. 70°N) at about monthly intervals between September 1979 and December 1983. Females generally spawned in September— October, and eggs hatched in the first half of April when maternal prawns were 48 months old. Recruitment was first evident in February at ca. 9 months of age (ca. 7- mm carapace length (C.L.)). An underlying cyclical trend of peaks and troughs was evident, affecting both younger and older prawns, probably indicative of seasonal emigration/immigration. C.L.-at-age varied by 10–50% between year classes, with a general tendency for better growth rates to be associated with periods of warming (from min. 1–2°C to max. 4–5°C) of the bottom waters: this also caused an increase in C.L. of 1–1.5 mm in the spawning stock. A single age class formed more than 90% of the spawning stock in any spawning season. The von Bertalanffy growth function (VBGF) has been fitted in its seasonally oscillating and standard forms to individual year classes and “pooled” data. Length-at-age was similar to that found for P. borealis stocks from the Atlantic waters of the southern Barents Sea. Instantaneous rates of total mortality were very high (annual Z of 1.9–2.1). probably owing to the combined effects of predation by cod and the prawn fishery.