Zooplankton of the Atlantic and Gulf Coasts: A Guide to Their Identification and Ecology

Fish Larvae

FISH LARVAE Ichthyoplankton, fish eggs and larvae, are common in estuaries and nearshore areas. Protected coastal regions and especially estuarine areas are critical spawning and/or nursery areas for most of the common commercial, sport, and forage fishes along our coasts. The larval stage is one of the most crucial and is, perhaps, the least understood of the developmental stages in the life history of most fishes. SPAWNING PATTERNS Most local bony fishes (teleosts) release eggs into the water where fertilization and subsequent development occur. Pipefishes, seahorses, and sea catfishes are notable exceptions since the males brood the young until their release as juveniles. Although the fish species that occur within the limits of our coverage area exhibit many reproductive strategies, most generally fall into one of the following general spawning patterns: 1.Adults spawn at sea, and early larvae are transported inshore by behaviorally mediated responses to currents as they grow (e.g., spot, left-eyed flounders, menhaden, pinfish). 2.Adults spawn close to the mouths of estuaries or in bays, and the larvae enter and grow in estuarine waters (e.g., red drum, weakfish). 3.Anadromous marine species migrate from the sea to spawn in fresh or almost freshwater areas of estuaries. The larvae move slowly into brackish waters as they grow (e.g., herrings, striped bass). 4.Catadromous freshwater fishes migrate as adults to the sea to spawn. Within our coverage area, the American eel is the singular representative of this pattern. 5.Resident estuarine fishes, including gobies and blennies, spawn in the middle reaches of estuaries. In larger estuaries, some residents undergo spawning migrations within the estuary itself (e.g., white perch to freshwater and hogchoker to more saline areas). 6.Resident species spawn across a wide range of salinities and conditions and complete larval development locally (e.g., mummichog, silversides). Spawning of most species is seasonal and is strongly influenced by changes in water temperature. The result is a temporal succession of larval fish abundance in each location with a predictable assemblage of species in each season. Of course, the timing and duration of spawning and larval recruitment varies between the northern and southern parts FISH LARVAE 311 of many species’ ranges, but general patterns of succession and species associations are recognized in most of the Atlantic and Gulf of Mexico. In winter to early spring, larvae of offshore spawners arrive in nearshore estuarine waters; this is known as larval ingress or settlement. Menhaden, spot, croaker, pinfish, and flounders (summer, southern, and gulf) exemplify this group. In summer, shallow coastal spawners, especially anchovies, gobies, and drums, produce larvae that dominate most estuarine ichthyoplankton collections. Each location along the coast has its own unique temporal sequence due to broad-scale latitudinal patterns of species occurrence, current conditions, local bathymetry (e.g., deep channels , shallow sounds, and broad intertidal flats), and habitat distribution (e.g., presence of marshes, oyster reefs, and sea grasses). In estuaries, the oligohaline interface between fresh- and saltwater is populated by anadromous fishes and by some larvae that migrate upstream from more saline spawning areas (e.g., gobies, hogchoker, and some drums). Anchovy , goby, and silverside larvae usually dominate middle reaches. The mouths of estuaries , coastal lagoons, and surf areas have their own characteristic and usually more diverse seasonal assemblages. The one constant feature of larval abundances in warm, temperate estuaries throughout the region is that in summer bay anchovy and goby larvae usually outnumber all other species combined in all but the freshest areas. Early larval stages of fishes are usually dispersed from spawning areas to other locations during developmental periods that often last from weeks to months. Because most larvae are relatively feeble swimmers, their redistributions are primarily determined by prevailing currents. As larvae grow and acquire more capacity to move relative to currents, they can facilitate their transport to more favorable locations. Behaviorally determined movements, including endogenous rhythms and responses to changing environmental conditions , have been noted for many larval fishes. Vertical movements can facilitate both transport and retention. The difficulties of studying microscopic larvae and physical characteristics of the water column at such a fine scale impede our understanding of the mechanisms of larval ingress and settlement. For instance, the relative roles of passive and active transport of larval fishes across the ocean shelf are largely unknown. Some of the general hydrological factors involved in the distribution, ingress, and settlement of larvae are covered in the introductory section, and movements for individual fish taxa are discussed in the text...