Journal of Fish Biology (1996) 48, 1139–1163
The multiple spawning pattern of weakfish in the Chesapeake Bay and Middle Atlantic Bight S. K. L-B*, M. E. C, J L. R. B* College of William and Mary, Virginia Institute of Marine Science, Gloucester Point, VA 23062, U.S.A. (Received 20 December 1994, Accepted 5 November 1995) Weakfish Cynoscion regalis were collected from commercial fisheries in the Chesapeake Bay and the Middle Atlantic Bight (n=4380) during 1989–1992 and their reproductive biology assessed using the gonadosomatic index, macroscopic gonad stages, oocyte diameter distributions, microscopic whole oocyte analysis and histology. Sex ratios were approximately 3 : 1, females to males, in 1990–1992. Most fish (90%) attained sexual maturity by age 1 and at a small size. Estimated mean length at first maturity was: 164 mm total length (TL) for males, and 170 mm TL for females. Weakfish spawn within the Chesapeake Bay, as far north as the Virginia/ Maryland border. Although spawning occurred during May–August and gonad development and initiation of spawning was synchronous, cessation of spawning was asynchronous. There was no indication that older fish exhibited a more extended spawning season than younger fish. Weakfish are multiple spawners with indeterminate fecundity. Oocyte development is asynchronous with oocytes of all stages being present in developed ovaries. Because of the complex and dynamic weakfish ovarian cycle, typical methods of assessing reproduction, such as the GSI and macroscopic gonad stages, are inadequate for this species if not used in conjunction with more detailed methods such as histology. ? 1996 The Fisheries Society of the British Isles Key words: sciaenidae; weakfish; Cynoscion regalis; spawning; reproduction.
INTRODUCTION The weakfish Cynoscion regalis (Bloch & Schneider) is a recreationally and commercially important sciaenid which is most abundant from North Carolina to New York, although it ranges from roughly eastern Florida to Massachusetts (Mercer, 1985). Most weakfish are believed to overwinter off North Carolina (Pearson, 1932), where they occur year-round. Due to their migrational pattern, weakfish occur in more northern waters only seasonally (Bigelow & Schroeder, 1953). In the spring, they migrate northward and inshore to estuarine feeding and spawning grounds, reversing this pattern in the fall (Wilk, 1979). Weakfish occur in Chesapeake Bay roughly from April through November (Pearson, 1941; Massmann et al., 1958), where they support one of the region’s most important fisheries (Rothschild et al., 1981). The reproductive biology of weakfish is not well understood. Although a number of studies have reported an extended spawning season (Hildebrand & Schroeder, 1928; Merriner, 1976; Shepherd & Grimes, 1984; Villoso, 1989), suggestive of a multiple spawner, descriptions of the weakfish spawning pattern are contradictory. Merriner (1976) suggested weakfish were multiple spawners in North Carolina waters, but Shepherd & Grimes (1984) found no evidence of this *Present address: University of Georgia Marine Institute, Sapelo Island, GA 31327, U.S.A. Tel.: +1-912-485-2143; fax: +1-912-485-2133; email:
[email protected]. 1139 0022–1112/96/061139+25 $18.00/0
? 1996 The Fisheries Society of the British Isles
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at the northern end of the range. Villoso (1989) observed multiple spawning in hormonally-injected weakfish in the laboratory and Taylor & Villoso (1994) reported apparent repeat spawning in Delaware Bay over a period of several days. Also, previous studies have assumed weakfish have determinate fecundity [i.e. potential annual fecundity is fixed prior to the onset of spawning (Hunter et al., 1992)] even though recent studies indicate many temperate and tropical fish have indeterminate fecundity (Hunter & Goldberg, 1980; DeMartini & Fountain, 1981; Hunter & Macewicz, 1985; Hunter et al., 1985; Horwood & Greer Walker, 1990). In these fish, unyolked oocytes are matured and spawned continuously and annual fecundity can be estimated only as the product of batch fecundity and the number of spawns per season (Hunter et al., 1985). In addition, weakfish reproduction has not been studied in Chesapeake Bay, although this area has long been considered an important spawning ground for this species (Higgins & Pearson, 1928; Merriner, 1973) and weakfish larvae are found commonly in spring and summer Chesapeake Bay ichthyoplankton (Olney, 1983). This study describes weakfish reproductive biology in the Chesapeake Bay and the Middle Atlantic region. Histology, the microscopic appearance of whole oocytes, the gonadosomatic index and macroscopic gonad stages were used to determine the type of spawning pattern (i.e. multiple or serial spawners versus total spawners) and the type of fecundity (i.e. determinate or indeterminate) as well as other reproductive parameters. A model of the weakfish ovarian cycle is then presented based on this analysis. MATERIALS AND METHODS COLLECTION OF SAMPLES A total of 4380 weakfish were collected from commercial fisheries in Chesapeake Bay and the Middle Atlantic region during 1989–1992. In 1989, fortnightly collections were made during the fishing season (April–October) from Chesapeake Bay pound net fisheries (Fig. 1). One 22·7 kg box of fish of each available market grade (small, medium or large) was purchased and processed for general biological data. In 1990, 1991 and 1992, weakfish were collected weekly during the fishing season for general biological data, primarily from gillnet and haul seine fisheries on the western shore of the lower Chesapeake Bay (Fig. 1). Fish alive when nets were emptied were marked as such. Since gillnets were set parallel to the shore, it was also possible to record the orientation of fish when gilled (i.e. caught moving inshore or offshore) to evaluate any trends in movement with gonad stage. Chesapeake Bay samples were supplemented during the winter (November–March) with monthly samples from commercial trawlers operating in Virginia and North Carolina shelf waters (Fig. 1), where weakfish are believed to overwinter (Pearson, 1932; Wilk, 1979). In 1991 and 1992, 52 mature fish were collected from Delaware Bay in April and May to evaluate if gonad development differed between Delaware Bay and Chesapeake Bay. Because commercial weakfish fisheries have a minimum size limit of 229 mm total length (TL), an additional 160 fish, 140–275 mm TL, were sampled during May–July 1991 and 1992 from the Virginia Institute of Marine Science juvenile trawl survey to supplement commercial samples for size and age at maturity data. GENERAL BIOLOGICAL DATA Total length was measured to the nearest mm and total gutted weight (TGW) was determined to the nearest gramme. Gutted weights were used, rather than total weights,
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F. 1. The Chesapeake Bay and Middle Atlantic Bight. -, Pound net, haul seine or gillnet collection sites; hatched area indicates where otter trawl collections of weakfish were made.
because weakfish are piscivorous and can swallow fish a third of their own weight, which would greatly bias somatic weights. Gonads were removed, sexed and gonad weight (GW) determined to the nearest gramme. Somatic weight (SW) was estimated as TGW"GW. Gonad appearance was documented and photographs taken of representative stages to verify later ovarian macroscopic gonad staging (Table I). Males were classified only as mature or immature, because a finer classification of male gonad stages is difficult and subjective. The gonadosomatic index (GSI) was calculated as
Ovaries ranging from small to medium (¦25% of body cavity); light orange in colour; no opaque (advanced yolked) oocytes present (mean GSI=1·81). Ovaries large (50–75% of body cavity); pale yellow in colour; opaque oocytes prevalent and easily detected; little ovarian vascularization and no signs of previous spawning (mean GSI=7·04). Ovaries ranging from medium to very large (25–100% of body cavity); clear (hydrated) oocytes visible amongst opaque oocytes, giving a speckled appearance; late in season, ovaries may be smaller and reddish due to an increase in the ratio of clear to opaque oocytes and ovarian vascularization (mean GSI=15·48). Ovaries ranging from medium to large (25–75% of body cavity); clear oocytes have been ovulated and are visible as a collective clear strip amongst the yolked oocytes; some may have been extruded; occasionally no opaque oocytes present (mean GSI=12·70). Ovaries somewhat flaccid, ranging from medium to small (¦30% of the body cavity); orangish in colour due to increased ovarian vascularization. Often a ‘ ridge ’ (a red area along the dorsal ovarian edge) is present. Remnant hydrated oocytes may occur in the ‘ ridge ’ or at the posterior end of the ovaries (mean GSI=6·63). Ovaries quite flaccid and small (90%) are primary growth; may have other oocytes in late stages of atresia; more follicular tissues than immature fish.
Primary growth and cortical alveoli oocytes present; yolked oocytes being resorbed. May be remnant hydrated oocytes or degenerating POFs.
Primary growth to advanced yolked oocytes present; often remnant hydrated oocytes and POFs present. May be atresia of advanced yolked oocytes. Similar to stage 3.
Primary growth to ovulated, hydrated oocytes and POFs present; may be major atresia of advanced yolked oocytes; occasionally only hydrated and primary growth oocytes present.
Primary growth to FOM/hydrated oocytes present; may be major atresia of advanced yolked oocytes; hydrated oocytes unovulated. Remnant hydrated oocytes from a previous spawn or degenerating POFs may be present.
Primary growth to advanced yolked oocytes present; may be atresia of advanced yolked oocytes; no remnant hydrated oocytes or POFs.
Only primary growth oocytes present; no atresia; ovarian membrane thin. Only primary growth, cortical alveoli and a few partially yolked oocytes present; there may be atresia.
Microscopic appearance
Macroscopic appearance refers to fresh ovaries. FOM, final oocyte maturation; GSI, gonadosomatic index; POF, post-ovulatory follicles.
7. Resting
6. Regressing
3*. Partially spent/ redeveloping
5. Running-ripe
4. Gravid
3. Fully developed
Ovaries very small, translucent, ribbon-like (GSI170 mm TL was indistinguishable from that of older, larger fish, but the percentage of age 1 spawners was consistently lower than the percentage of age 1 females in overall yearly samples (Table III). Mean lengths of age 1 females in May and June were 176 mm TL (n=42) and 200 mm TL (n=41), respectively, with many females occurring which were smaller than the L50 value of 170 mm TL. SPAWNING SEASON AND LOCATION
Weakfish spawn over an extended period, roughly from May through August. Mean monthly GSIs for both males and females were low during January– March (¦1 for males and
