Batch Fecundity in Multiple Spawning Fishes

Abstract

Methods for estimating batch fecundity are described, including identifmtion and sampling the eggs that constitute one spawning batch within the ovary. Using northern anchovy, Engaulis mordar, as an example, methods are developed for evaluating the accuracy and precision of batch fecundity estimates. Included in this analysis are the number and location of ovarian tissue samples, optimal numbers of females, and selection of the appropriate function to express the relation between batch fecundity and female weight. We found for anchovy that the optimum number of ovarian tissue samples wps 2-3 per ovary and that to keep the coemcient of variation for the mean fecundity of population under 10% required a sample of 50 or more females. Analysis of covariance indicated that the batch fecundity of northern anchovy varied significantly among years (1951-84) indicating that the relation between female weight and fecundity must be newly eslablished each year. INTRODUCTION A key issue in the estimation of fecundity of oviparous fishes is whether or not the annual fecundity can be estimated from the standing stock of advanced oocytes in the ovary prior to the onset of the reproductive season. In some boreal species, frequently called total or isochronal spawners, all the eggs to be released in a season develop synchronously prior to spawning (hence the term isochronal) and spawning typically takes place over a short period (Holden and Raitt 1974). In such species, the standing stock of oocytes within a certain range of maturity classes is considered to represent the annual fecundity of the spawner. The groups of oocytes to be spawned in the season are usually identifiable because a distinct hiatus in oocyte maturity classes exists between the small, immature, unyolked oocytes that occur the year around and the synchronously maturing annual batch (Hickling and Rutenberg 1936; Yamamoto 1956). Although some of these fishes may spawn repeatedly during the season, for example, whiting and haddock, the standing stock of yolked eggs is considered representative of the annual fecundity (Hislop 1975; Hislop et al. 1978; Hislop, pers. commun.l). An exception to this occurs when unfavorable conditions result in resorption of some of the advanced eggs in the ovary at the end of the season. The extent of this potential bias (overestimation of annual fecundity) is unknown. In many temperate and tropical fishes (frequently called multiple, partial, serial, or heterochronal spawners), annual fecundity is seasonally indeterminate and batch fecundity is the only useful measurement. In such fishes the standing stock of yolked eggs, regardless of maturity state, give no indication of annual fecundity because these fishes continuously mature new spawning batches throughout a typically protracted spawning season. In the active ovaries of fishes with indeterminate annual fecundity, the oocytes usually occur in nearly all maturity stages; they range in size continuously from small unyolked oocytes <0.1 mm diam. to yolked oocytes 0.4-0.7 nun diam., and no large hiatus exists between maturity classes of oocytes except for one between hydrated oocytes and advanced yolked oocytes which is of a temporary nature. Such fishes usually spawn many times during a season. The northern anchovy spawns at 7-10 d intervals for 2 or 3 mo and averages 20 spawnings per yr (Hunter and b o n g 1981). and the scianid, Seriphus politus, has a similar reproductive output (DeMartini and Fountain 1981). Thus, for these fishes, identification of a predetermined annual spawning batch is a hopeless exercise, and the only useful fecundity measurement is the number of eggs produced in a single spawning batch (batch fecundity); annual fecundity is a function of both the batch fecundity and the number of spawnings per year. Spawnings are so numerous in these fishes that small unyolked oocytes <O.l nun diam. would have to mature in a season to account for the number of spawnings (Hunter and Leong 1981). The standing stock of oocytes is occasionally used to estimate annual fecundity in such common fishes as Scomber, Trachurus, and Merluccius, which by the standard criteria have indeterminate fecundity. That annual fecundity is predetermined in such fishes is an assumption with little or no Supporting evidence. The criteria and approaches for distinguishing between determinate and indeterminate fecundity are discussed in greater detail (Hunter and Macewicz 1985). The objective of this paper is to describe the methodologies for estimating batch fecundity in fishes with indeterminate seasonal fecundity. We do not consider the well documented methodology 'John R. G. Hislop, DAFS Marine lab . , P.0 Box 101, Victoria Road, Aberdeen AB9 8DB. Scotland, pen. commun. Oct. 25, 1983