Spawning Site Selection by Coastal Fishes in the Northern Gulf of Mexico
Donald M. Baltz
Overall Research Goal
To develop basic ecological information on the habitat requirements of spawning soniferous fishes that can be used to study their breeding behavior and understand environmental factors that influence their spawning success and the successful recruitment of early life history stages into nursery habitat.
Passive acoustics can contribute to our understanding of how fishes use their environment for important life history functions. For the family Sciaenidae, which includes more than 30 species of croakers, drums, seatrout, etc., in US coastal waters, males generally gather in the late afternoon or early evening and drum to attract females in spawning condition to aggregation sites. These sites are apparently selected by males because they are suitable for the fertilization, survival, and dispersal of early life history stages (i.e. eggs and early embryos). Simple hydrophones can be used to locate and identify spawning aggregations of species of interest and also to learn about the daily and seasonal patterns of reproductive activity. Combined with other scientific instruments, researchers can use hydorphones to describe the time and place of spawning in terms of important environmental variables such as depth, substrate, salinity, oxygen concentration, turbidity, light intensity, water temperature and current velocity. Careful analysis of this suite of variables tells us what is important to the fish in selecting sites for spawning.
We use a microhabitat approach to make multiple, randomly selected, independent observations with hydrophones at times and places where spawning is suspected (Figure 1). We also record environmental conditions and make short tape recordings for laboratory analyses. When suspected spawning sites are located, we collect water samples to gather eggs for rearing to an identifiable stage to verify spawning. Comparison of all randomly selected sites (all sites) with those used for spawning helps us to identify the environmental conditions that are most important to the fish (Figure 2). A suitability analysis offers valuable insights into the range along environmental gradients (e.g.water temperature) that are most essential for successful spawning.
A variety of important Sciaenid fishes are found in the coastal waters of the northern Gulf of Mexico and the spawning requirements of most of them have not been well studied. One reason for this is that many of them spawn late in the evening in open waters that are often not safe for small boats. Most notably, the red drum (Sciaenops ocellatus) spawns on Louisianas coastal shelf amid a dynamic landscape that is experiencing natural and anthropogenic changes due to a variety of factors, including water management, sea-level rise, canal dredging, industrial development, land loss, shipping, and the effects of fishing. For this and other species, we need to know what ecological conditions along complex environmental gradients in our coastal water are most suitable for spawning so that these essential habitats can be identified and protected. My wish list is topped by a fixed or moveable listening array with overlapping directional capabilities to generate position fixes, computer programs to process fix data, and real-time transmission capabilities to allow relatively small boats to move to aggregation sites and random sites for measurements of resource use and availability.
Past and Current Studies
Baltz, D. M. and J. A. Campos M. 1996. Hydrophone identification and characterization of Cynoscion squamipinnis (Perciformes: Sciaenidae) spawning sites in the Gulf of Nicoya, Costa Rica. Revista de Biologia Tropical 44(2):743-751.
Saucier, M. H., D. M. Baltz, and W. A. Roumillat. 1992. Hydrophone identification of spawning sites of spotted seatrout (Cynoscion nebulosus) near Charleston, South Carolina. Northeast Gulf Science 12(2):141-145.
Saucier, M. H. and D. M. Baltz. 1993. Spawning site selection by spotted seatrout (Cynoscion nebulosus) and black drum (Pogonias cromis) in Louisiana. Environmental Biology of Fishes 36:257-272.