Abundance, dynamics and mortality of the Delaware Bay stock of blue crabs, Callinectes sapidus

Journal of Shellfisheries Research, Jan, 2005 by Desmond M. Kahn, Thomas E. Helser

ABSTRACT The Delaware Bay stock of blue crabs supports a bistate fishery in New Jersey and Delaware, with annual landings climbing through the 1980s and 1990s to almost 11 x [10.sup.6] pounds (4,390 metric tons) in 1995 and then declining to a recent average of 7 x [10.sup.6] pounds (2,796 metric tons) over the last 5 y. In Delaware, this fishery ranks as number one in value. Landings declines in 1996 spurred efforts to conduct a stock assessment, which is now updated annually. This assessment was based on: (1) a biomass-based minimum recruitment threshold from a Ricker stock-recruitment model fit to indices of relative abundance from a research trawl survey and (2) a catch-survey model incorporating observation and process error that produced annual estimates of absolute abundance, biomass, and fishing mortality rates from 1979 through 2002. Adult blue crab abundance estimates showed a positive trend over the period, ranging from 20 x [10.sup.6] in 1979 up to 146 x [10.sup.6] in 1993, with recent estimates between 70 x [10.sup.6] and 97 x [10.sup.6]. Estimated average exploitable stock biomass over the period was 23.43 x [10.sup.6] pounds (9,357 metric tons). Recruit abundance was highly variable, ranging from 34 x [10.sup.6] up to 631 x [10.sup.6]. Use of the log survival ratio to estimate Z showed no trend in Z, although estimates were highly variable. Estimation of the exploitable stock size was problematic due to high density-dependent recruit mortality. Because of this fact, we developed upper and lower bounds of the exploitation rate, then estimated upper and lower bounds of F from Baranov catch equation, F = [micro]/(1 - [e.sup.-Z])*Z. We also estimated the Collie & Kruse (1998) harvest rate and extended it to estimate F. The upper bound of F ranged from 0.13 up to 0.77 and averaged 0.44. The upper bound on F and the Collie-Kruse F showed a positive linear or curvilinear trend. Annual M estimates from Z - F, conditioned on an original model input value of constant M = 1.0, were erratic and showed no trend but were correlated with recruitment, supporting the hypothesis of compensatory density dependence. The relatively low estimate of F versus M and the overcompensatory and resilient stock-recruitment relationship suggest that overfishing is not occurring on this stock.

KEY WORDS: Delaware Bay, blue crabs, Callinectes sapidus, catch-survey model, density, dependent mortality, compensatory mortality

INTRODUCTION

The blue crab (Callinectes sapidus Rathbun) inhabits estuaries from southern New England to Uruguay (Williams 1984). The center of its distribution is tropical; severe winters can kill significant proportions of overwintering blue crabs from Chesapeake Bay north (Sharov et al. 2003, Kahn et al. 1998). Juveniles and adults primarily remain within one estuary for life, although tagging work has shown that adults may move into nearby coastal waters (Fischler & Walburg 1962, S. McKenna, North Carolina Division of Marine Fisheries, pers. comm.).

The peak hatching of larvae in Delaware Bay occurs in July. After hatching, larvae are carried onto the continental shelf along the North American Atlantic coast and into the Gulf of Mexico along the Gulf coast, where larval development occurs (Epifanio 1995, Perry et al. 1998, Epifanio & Garvine 2001). Stock mixing occurs at this stage; the current hypothesis for Delaware and Chesapeake Bays, however, is that larvae travel in an oval or circular trajectory on the shelf and can re-enter their estuary of origin (Epifanio 1995, Garvine et al. 1997, Epifanio & Garvine 2001). Garvine et al. (1997) concluded that the Delaware Bay stock was probably the primary source of its own recruits, agreeing with Epifanio et al. (1984). The mouth of Chesapeake Bay is 160 km to the south, so it is not likely a dominant influence on Delaware Bay recruitment. For both estuaries, larvae travel south initially along the shore, then are pushed offshore and northward via Eckman flow from the dominant southwesterly winds of late summer and early fall. If the larvae wind up off the mouth of Delaware or Chesapeake Bay, they can re-enter in large numbers when northeasterly or northerly wind events push large volumes of water into estuaries via Eckman flow, according to Garvine et al. (1997) and Epifanio (1995). Other stocks in smaller estuaries north and south of Delaware Bay contribute larvae to the Bay to a greater or lesser extent (Garvine et al. 1997). Peak settlement occurs from late summer to early fall in Delaware Bay (Kahn et al. 1998). The Delaware Bay blue crab fishery is shared between the states of New Jersey and Delaware. The Bay is split longitudinally between the two states along the shipping channel, in contrast to the division of the Chesapeake Bay between Virginia and Maryland, which assigns the lower Chesapeake to Virginia and the upper Chesapeake to Maryland.

Despite the fact that Delaware Bay is near the northern extent of the species range, blue crab catches produce the largest dockside value of any fisheries resource in Delaware. The blue crab fisheries in Maryland, Virginia, and North Carolina provide the highest value in those states as well. Reliable Delaware landings data begin in 1973, whereas reliable New Jersey landings begin with 1978 (Fig. 1). In 1977, Delaware landings dropped sharply to less than 800,000 pounds, approximately 26% of 1976 landings. The winter of 1977 was the most severe in recent decades and was followed by another severe winter in 1978. Both these winters undoubtedly caused major winterkill of mature crabs. Landings in Delaware and New Jersey have followed similar patterns. After the decline in 1977, combined landings remained low for 8 y, averaging about 1.5 million lbs. Not until 1985 did Delaware landings return to the level of the mid 1970s. Landings climbed to the peak of 10.8 million lbs. in 1995, then declined in 1996 after a severe winter, when the Delaware Division of Fish and Wildlife documented winterkill. Dredge samples in March 1996 found 47% mortality, primarily of mature females. Landings then rebounded, but have not returned to the levels of the mid 1990s.