Settlement is arguably the last stage of high mortality in the life cycle of many marine fish species with a bipartite life cycle, making the number of larvae that settle successfully to a benthic habitat a reasonable determinant of future population size. Habitat selection during settlement is likely an active process, however, much of what we know about settlement behavior comes from studies in coral reef ecosystems. This collection of studies examined the ability of estuarine-dependent fish larvae (red drum, Sciaenops ocellatus) to locate settlement sites based on information received from their senses, with a focus on the different spatial scales over which larval sensory systems operate. During the pelagic phase, red drum are exposed to elevated sound levels in the tidal inlets. This noise caused larvae to reduce their activity in the form of fewer turns, less time spent swimming, and a lower overall mean speed compared with silent controls. As red drum approached settlement size, but not at earlier stages, they responded to olfactory cues associated with seagrass beds, their primary settlement habitat. Activity increased in the presence of lignin, a compound associated with the cell walls of vascular plants. Also, settlement-size larvae spent more time in water masses taken from seagrass beds compared to control sea water, implying a preference for olfactory cues associated with estuaries. While settlement and post-settlement-size larvae positioned themselves near the benthos regardless of water column height or substrate color, pre-settlement larvae moved away from yellow and green (associated with the estuary) but not black (associated with a deep water column) benthic colors. Additionally, red drum larvae settled to seagrass and sand at a smaller size than they did to oyster shells, and they delayed settlement when a suitable benthic habitat was not available. These findings can be interpreted as evidence for an estuarine-dependent species taking advantage of cues available to multiple sensory systems in order to actively locate settlement habitats. / text

While substantial progress has been made in replacing live prey with artificial diets in the feeding of marine fish larvae, it still remains impossible to successfully rear larvae on artificial diets without some period of co-feeding live prey or algae. This study investigated the presence and role of the gastrointestinal hormone cholecystokinin (CCK) in the red drum (Sciaenops ocellatus) to gain a better understanding of the factors limiting the utilization of artificial diets by red drum larvae. Work with other fish species has shown that CCK is the principal hormone which regulates the release of pancreatic enzymes into the gut lumen and emphasizes the potential importance of CCK in early red drum larvae. This work investigated the hypothesis that some signal present in the live prey or algae stimulates CCK and thereby initiates the digestive process in the larvae. First, the nucleotide and amino acid sequence of the putative red drum cholecystokinin precursor was determined and the development of CCK immunoreactive cells was examined. This work showed that red drum CCK is highly similar to CCK in other vertebrates and can be detected in the digestive tract of larval red drum within three days after the initiation of exogenous feeding. Next; postprandial trypsin, CCK, and CCK mRNA responses were quantified in red drum juveniles and larvae over a three hour period. Both CCK and trypsin were increased within thirty minutes following feeding while CCK mRNA levels were increased within the next two to three hours. Finally, the trypsin, CCK, and CCK mRNA responses of red drum larvae to homogenates of live prey and algae were examined. Homogenized rotifers appeared to be sufficient to induce both the CCK and trypsin responses in larval red drum. These results suggest that in addition to other factors, some component of live prey may initiate the release of CCK and prime the digestive process. Understanding these factors and their effects in early larvae may allow us to formulate and produce a prepared diet which will support growth and survival to metamorphosis equal to that provided by live feeds. / text

Bayou St. John is a degraded water-body located within the City of New Orleans and is the focus of restoration efforts. I tested the ability of reintroduced red drum (Sciaenops ocellatus) to survive in this system and assessed their habitat use and behavior. I tracked 29 red drum fitted with external acoustic radio transmitters to determine if they could survive the degraded habitats and determine their general dispersion within the Bayou. All 29 tagged red drum exhibited post-stocking movement (i.e., survival) and occurred primarily in the northern section of the Bayou (nearest Lake Pontchartrain). To assess habitat use and behavior on a finer scale, a second group of 19 red drum were internally tagged with VEMCO transmitters and movements monitored by four remote receivers. These fish exhibited behavior similar to red drum in natural habitats. Monthly movements changed as temperatures changed but were not influenced by diurnal differences, salinity, or conductivity.

Bayou St. John (BSJ) and City Park Lakes and Lagoons (CPLL) are urban waterways in New Orleans, Louisiana. I studied habitat selection of red drum in BSJ, and fish assemblage change in BSJ and CPLL over 40 years. Temperature was found to be the best predictor of red drum habitat selection in Bayou St. John, while salinity and change in depth also were found to be good predictors for certain sites. Potential prey item abundance did not appear to influence habitat selection. Using data from 1971 – 2010, nearshore habitats in CPLL were affected by Hurricane Katrina, but have sense recovered and nearshore habitats in BSJ were found to have decreased diversity. Pelagic habitats in both areas were found stable across 40 years. Since 2006, nearshore assemblages were similar for CPLL and BSJ with a decrease in fishes from Order: Cyprinodontiformes and an increase in other fishes seen across years.

This collection of studies was designed to understand the mechanisms and consequences of competition among early life stages of red drum (Sciaenops ocellatus), with a special attention to asymmetric competition and multiple-breeding strategy of parents. The overall hypotheses were that (1) red drum larvae show behavioral interactions, and the magnitude of these interactions is explained by the sizes of the competing individuals, (2) red drum larvae compete for food and habitat use, and the competition is asymmetric as determined by size and behavioral interactions, and (3) the parents can reduce negative effects of competition among larvae and increase larval survival by altering their multiple-spawning traits. The laboratory experiments showed the existence of sociality in red drum larvae. The magnitudes of aggressive behavior, vigilant behavior and shoaling behavior were explained by a combination of absolute and relative phenotypes of competing individuals, such as body lengths and body condition. A new method of combining the laboratory experiments and the foraging theory disentangled interference competition for food from exploitative competition, revealing that both absolute and relative body lengths of competing individuals influence feeding efficiencies. In addition, vigilant behavior decreased feeding efficiency regardless of body sizes. The competitor size and behavioral interactions between individuals had different effects on competition for habitat use: the existence of larger individuals prevented the newly settled larvae from entering preferred habitats, whereas the aggressive behavior from the occupants of preferred habitats moved newly settled larvae to the unpreferred habitats. Finally, computer simulations using an individual-based model revealed that as interference competition among larvae increased, the number of spawning events and the spawning interval of the parents increased to lessen competition and maximize total survival of offspring. / text