Seasonal and ontogenetic changes in the vertical distribution of eggs and larvae of mackerel (Scomber scombrus L.) and horse mackerel (Trachurus trachurus L.)
Introduction
Information on the vertical distribution of fish eggs and larvae is an essential pre-requisite for efficient sampling, e.g. for stock estimation based on plankton egg surveys (Lockwood et al., 1981). Equally, a knowledge of their vertical distribution is required for studies of transport and survival of eggs and larvae in relation to current systems and biological processes in the water column. In the latter context, an EU funded programme (SEFOS — Shelf-Edge Fisheries and Oceanography Studies) has been designed to study the effects of the shelf-edge current on fish stocks from Portugal through Biscay and west of the British Isles to Norway. One aspect of the SEFOS programme was an investigation of the distribution and advection of the planktonic stages of mackerel (Scomber scombrus) and horse mackerel (Trachurus trachurus) in relation to their subsequent recruitment. The present work on the vertical distribution of the eggs and larvae is a part of this study.
Spawning of both mackerel and horse mackerel is widespread over much of the European shelf, extending from the North Sea and west of Scotland as far south as the coasts of Spain and Portugal (Lago de Lanzós et al., 1993; Anon., 1994) and, for horse mackerel to waters off West Africa (Eaton, 1983; John et al., 1991). For both species the highest incidence of spawning is at the shelf-edge and over adjacent shelf regions of the Celtic Plateau and Biscay, the main spawning season in these areas for mackerel being from mid-April to mid-June (Anon., 1993, Anon., 1996), and for horse mackerel somewhat later, from May to July (Eaton, 1989; Franco et al., 1993; Anon., 1996).
Reports on the vertical distribution of mackerel eggs west of the British Isles have shown that once thermal stratification is present they are distributed mostly in the upper mixed layer above a depth of about 100 m (e.g. Coombs et al., 1981, Coombs et al., 1990; Anon., 1993). A similar but more extreme restriction of eggs to the upper 10 or 20 m of the water column has been observed for mackerel eggs in other regions when stratification is well developed (e.g. North Sea — Iversen, 1977; Coombs et al., 1981; English Channel — Southward and Barrett, 1983; east coast of North America — Sette, 1943; Lafontaine and Gascon, 1989). More limited information on their depth distribution in the absence of any appreciable thermal stratification has indicated that under such conditions, eggs are found at greater depths (Walsh, 1976; Coombs et al., 1981; Röpke, 1989). Larvae have been reported consistently as being in the upper mixed layer (e.g. Sette, 1943; Coombs et al., 1981, Coombs et al., 1983, Coombs et al., 1990; Southward and Barrett, 1983; Ware and Lambert, 1985; Lafontaine and Gascon, 1989; Röpke, 1989).
Less information is available on the vertical distribution of eggs and larvae of horse mackerel and much of this is derived from widely distributed sites away from the main spawning grounds along the northern European shelf-edge. However, there is a consistency in the observation of increased abundance of both eggs and larvae towards the surface (e.g. off Portugal — John and Ré, 1993; off West Africa — John, 1985; John et al., 1991; in the English Channel — Russell, 1930; Southward and Barrett, 1983; at the shelf-edge in Biscay — Coombs et al., 1979). Concentrations of eggs and larvae in the neuston have also been noted (e.g. in the North Sea — Nellen and Hempel, 1970; in the Black Sea — Zaitsev, 1971). A similar near-surface habit has been described for the early developmental stages of other Trachurus species (e.g. Trachurus symmetricus off California — Ahlstrom, 1959; Trachurus trachurus capensis in the northern Benguela system — Olivar, 1990).
However, few of the above sets of results have included sufficient coverage for a comprehensive description or statistical analysis of the depth distribution of eggs or larvae. The purpose of the present paper is to summarise the results from sampling to the west of the British Isles and to provide a more complete description of their vertical distribution. Preliminary accounts were given in Coombs et al., 1996a, Coombs et al., 1996b.
Section snippets
Materials and methods
Sampling was carried out at the shelf-edge and adjacent areas west of the British Isles and in Biscay between March and June on various cruises between 1974 and 1991 (Fig. 1 and Table 1). A more detailed set of samples was obtained over a 33 h period in June 1995 at a site in the Porcupine Seabight to the southwest of Ireland (Fig. 1 and Table 1).
All sampling was carried out using versions of the Longhurst-Hardy Plankton Recorder (Williams et al., 1983; see also Pipe et al., 1981) which takes a
Results
There was a progressive change in the mean depth of mackerel eggs over the spawning season (F3,83=22.86, p=0.0001). In March and April, eggs were distributed over a wide depth range to at least 400 m depth, while in May and June they were in shallower depths, mostly in the upper 50 m of the water column (73.0% of eggs in the 0–50 m depth range in May and 81.6% in June; Fig. 2, Fig. 3). These changes in vertical distribution corresponded to development of the seasonal thermocline, little vertical
Discussion
The observed relatively near-surface distribution of mackerel and horse mackerel eggs and larvae, at least in May and June, is typical of ichthyoplankton in general (e.g. Boehlert et al., 1985; Conway et al., 1997). The relatively deep distribution of mackerel eggs, as observed in March and April (>200 m depth) is less common, and usually confined to habitually deep-water species, or to those found at depth during the spawning season (e.g. blue whiting, Micromesistius poutassou, Coombs et al.,
Acknowledgements
The authors gratefully acknowledge the assistance given by the following organisations to participate in their cruises: Fisheries Laboratory Lowestoft, Marine Laboratory Aberdeen, Institute for Marine Research Bergen, Institut für Meereskunde Kiel, Institut für Seefischerei Hamburg and Station Biologique Roscoff. Funding for the work has been provided in part under EU contract AIR2-CT93-1105.
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