Factors contributing to variability in larval ingress of Atlantic menhaden, Brevoortia tyrannus

Abstract

Annual recruitment levels of age-0 juvenile Atlantic menhaden to Chesapeake Bay, which historically supported >65% of coastwide recruitment, have been consistently low since the 1980s. Diminished larval supply to the Bay is one hypothesized explanation. In a three-year ichthyoplankton survey at the Chesapeake Bay mouth, abundance of ingressing larvae varied nine-fold among years. Larvae were most abundant in 2007–2008 and less abundant in 2005–2006 and 2006–2007. High month-to-month variability in larval concentrations was attributable primarily to seasonality of occurrences. There was no defined spatial pattern in distribution of larvae across the 18-km-wide Bay mouth, but larvae at the south side were longer and older on average than larvae at the middle and north side. Environmental variables measured at the times of larval collections were not correlated consistently with temporal and spatial variability in abundance of larvae at ingress, highlighting complexity and suggesting that abundance may be controlled by processes occurring offshore during the pre-ingress phase. Moreover, the substantial differences in inter-annual abundances of larvae at the Bay mouth were not concordant with subsequent abundances of age-0 juveniles in the three survey years, indicating that important processes affecting recruitment of Atlantic menhaden operate after ingress, during the larval to juvenile transition stage.

Introduction

Recruitment levels of age-0 juvenile Atlantic menhaden (Brevoortia tyrannus) in the Chesapeake Bay have been low since a decline in the mid-1980s (MDSG, 2009; ASMFC, 2010). Historically, the Bay contributed >65% to the coastwide recruitment of this abundant species that supports the largest single-species fishery on the Atlantic coast of North America (ASMFC, 1999; MDSG, 2009). Several hypotheses have been proposed to explain low recruitments (ASMFC, 2004). Among them is a decline in the supply of larval Atlantic menhaden from offshore waters where they begin their lives. Recruitment of Atlantic menhaden in the Chesapeake Bay depends on spawning by a single coastwide population that ranges from Nova Scotia to Florida and spawns on the continental shelf throughout most of its range (Hildebrand, 1964; Reintjes, 1969; MDSG, 2009).

During summer, Atlantic menhaden adults are distributed throughout its range, with older age classes predominating to the north and younger ages to the south (June and Reintjes, 1960; June, 1961; June and Nicholson, 1964; Rogers and Van Den Avyle, 1989). Higham and Nicholson (1964) noted that spawning occurs in every month of the year. These authors and others (Kendall and Reintjes, 1975; Judy and Lewis, 1983) suggested that, because most of the oldest Atlantic menhaden are found in northern reaches during summer, along with simultaneous occurrences of eggs and larvae, summer spawning must occur primarily in the northern part of Atlantic menhaden's range. By early fall the adult component of the population begins to migrate southward (Roithmayr, 1963; Reintjes, 1969; Nicholson, 1971; Dryfoos et al., 1973). Spawning then intensifies along the mid-Atlantic coast and in the South Atlantic Bight (Fig. 1) and was proposed to peak when adult Atlantic menhaden reach waters south of Cape Hatteras in December (Higham and Nicholson, 1964; Judy and Lewis, 1983).

Atlantic menhaden eggs hatch offshore and larvae are transported to estuaries prior to transformation to the juvenile stage (Ahrenholz, 1991). In the offshore environment, larval fishes have limited swimming capability (Shanks, 1995) and thus are largely dependent on ocean circulation for transport to estuaries (Epifanio and Garvine, 2001). Modeling in the South Atlantic Bight Recruitment Experiment (SABRE) provided insights into possible source regions and transport fates of shelf-hatched Atlantic menhaden larvae (Quinlan et al., 1999; Rice et al., 1999; Stegmann et al., 1999; Werner et al., 1999). Research on duration of the transport period, based on estimated age-at-ingress, indicated that most Atlantic menhaden larvae ingressing to estuaries in North Carolina had transport durations of nearly two months (Warlen, 1992, 1994; Warlen et al., 2002). It has been hypothesized that inter-annual differences in offshore environmental conditions may lead to variable survival of Atlantic menhaden larvae that partly accounts for variability in supply to estuaries (Lewis, 1965; Powell and Phonlor, 1986).

The supply of offshore-spawned larvae to estuaries is an important feature in the life cycles and in the recruitment of many fishes (Miller et al., 1984; Hettler and Hare, 1998). Variability in the shoreward delivery of larvae from offshore spawning grounds and environmental or hydrographic factors at ingress into estuaries add to the complexity of understanding ingress dynamics and recruitment (Joyeux, 1998; Hare et al., 2005; Sullivan et al., 2006; Braverman et al., 2009; Love et al., 2009).

In North Carolina estuaries, peak ingress of Atlantic menhaden occurred from February through April (Lewis and Mann, 1971; Hettler and Chester, 1990; Hettler et al., 1997). Prior to our research, little was known about periods of peak ingress into Chesapeake Bay. Our objectives were to determine inter-annual and monthly abundances, and factors associated with variability in patterns of ingress by Atlantic menhaden larvae at the Chesapeake Bay mouth. Based on ichthyoplankton surveys conducted during months believed to represent the peak ingress period, ingress and its variability are described and analyzed in relation to tide stages, time of day, depth distributions and location across the Bay mouth. Ingress abundances in the three-year program were compared with estimates of age-0 juvenile abundances to determine if larval and juvenile abundances were concordant.