Behaviour and sensory physiology of Atlantic menhaden larvae, Brevoortia tyrannus, during horizontal transport

Abstract

Atlantic menhaden, Brevoortia tyrannus, spawn on the continental shelf off North Carolina in the late fall and winter. Larvae are transported shoreward where they enter and migrate up estuaries to the juvenile habitat. In this paper, we synthesize behavioural and sensory physiological studies of the ontogeny of biological rhythms, responses to temperature, salinity and light, and swimbladder inflation and deflation of this species, to develop descriptive models of larval behaviour in ocean and estuarine areas that contribute to horizontal transport. Young (postyoke sac, <9 mm total length, TL) larvae occur primarily offshore, have an endogenous rhythm in nocturnal diel vertical migration, and are predicted to remain at depths above the pycnocline, if present, due to responses to light, temperature and salinity. They ascend in response to a salinity increase and temperature decrease, which frequently occur at the pycnocline. Intermediate age (9–17 mm TL) and older (18–27 mm TL) larvae are found in inshore and estuarine areas. They have an endogenous activity rhythm that suggests they will remain at moderate depths during the day in the ocean. At sunset, they swim up to surface to engulf air to fill their swimbladders and then sink due to inactivity and negative buoyancy. Thus, at night the majority of larvae are deeper than during the day. The swimbladder is deflated at sunrise in response to light and larvae are predicted to swim up to their daytime depth. They have the same responses to a temperature decrease and salinity increase as younger larvae, which should keep them above a pycnocline. The ascent to fill the swimbladder is not impeded by responses to temperature or salinity change. Chemical cues in ocean and estuarine waters induce different behavioural responses. Larvae use selective tidal stream transport for up‐estuary movement in which they are in the water column on rising tides at night and not abundant at other times. Larvae lack an endogenous tidal rhythm in vertical migration upon entering an estuary, and swimming during the day is reduced by light. The ascent into the water column during flood tides may be cued by the rate of increase in salinity at this time. Collectively, these behaviours contribute to feeding, predator avoidance and transport from the spawning area to the juvenile habitat.