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Halichoeres socialis Randall & Lobel, 2003, is the only wrasse species (Teleostei: Labridae) with a restricted distribution within the Caribbean Meso-American Barrier Reef (MABR), Central America. This study examined the early life history of H. socialis to gain insight into its natural history and to assess if any pattern in otolith microstructure could be linked to its endemism. The sagittal otoliths from 67 individuals (ranging 9.8–43.4mm SL) were analyzed to determine the daily age and increment profile. Results indicate that (1) H. socialis displays a typical “settlement mark”; (2) the pelagic larval duration (PLD) ranged from 22 to 32 (mean=27, n=59) days, which was similar to its congeners; and (3) the juvenile growth rate is 0.7mm/d, which was faster than most congeners. The endemism of H. socialis is not explained by its PLD. Alternatively, it is possible that larval behaviors (vertical migration and/or schooling) play a key factor in how H. socialis larvae are retained within the MABR region.

Temperate fishes thrive in dynamic thermal environments, where water temperature can vary dramatically across days, seasons, and years. To survive in these variable environments, they must acquire essential energy and nutrients to support their physiological responses to temperature change. However, climate change is expected to impact the nutritional landscape (prey options, quality, and quantity) for many marine animals, which may limit their ability to respond effectively to co-occurring environmental changes, like temperature rise. In my PhD thesis, I studied how an abundant kelp forest fish, opaleye (Girella nigricans), is affected by simultaneous changes in their diet and environmental temperature.One way opaleye may respond to temperature change is by adjusting their diet selection or ingestion rate to modify their nutritional status. However, my results demonstrate that diet and temperature have inconsistent effects on the thermal sensitivity of ecologically important traits and that opaleye are prone to making diet choices that have negative impacts on their performance in warm water. I also explored the untested hypothesis that nutritional status can affect the rate of thermal plasticity but found that opaleye’s rate of plasticity was insensitive to diet. However, their capacity for plasticity was diet-sensitive and could be partially explained by their fatty acid assimilation. Finally, I found that opaleye energetics and thermal tolerance are impacted by the combined effects of food restriction and dietary fat composition, where the quality of the fish’s diet can rescue them from the harmful effects of food restriction. Overall, this dissertation demonstrates that diet mediates thermal plasticity in ectotherms, like opaleye.

Rapid human-driven environmental change, which is affecting nearly every ecosystem worldwide, holds great potential to negatively affect biodiversity by exposing populations to evolutionarily novel environmental conditions. Faced with a rapidly changing environment, individuals and populations will disperse, adapt, acclimate, or go extinct. Specifically, climate change via water warming holds great potential to affect aquatic organisms through its effects on individual energy budgets. Lake Tanganyika, an East African Rift Lake that supports a diverse and highly endemic fish assemblage, has been warming at an unprecedented rate during the past century and is expected to continue to warm over the next century. Herein, I use a controlled laboratory experiment to assess the acclimation potential of the endemic Lake Tanganyika cichlid, Julidochromis ornatus, to chronic elevated temperatures that are expected to occur by the start of the next century. Using measures of metabolic rate, somatic growth, and reproductive life history, I found that J. ornatus may not be able to cope with expected warming through acclimation occurring during the adult life stage. When compared to individuals exposed to the current Lake Tanganyika temperature of 25°C in a control treatment, adults exposed to 29°C for ~6 mos exhibited elevated routine metabolic rates, reduced somatic growth (in terms of mass but not length), and decreased reproductive rates (number of broods produced per breeding pair per day). These findings suggest that J. ornatus adults do not exhibit the capacity for metabolic or reproductive acclimation. Thus, unless J. ornatus, and other fishes like it, have the potential for developmental or transgenerational acclimation, the future persistence of this species will depend on either its adaptive potential, or its ability to disperse to more suitable thermal habitat.

Invasive aquatic species are becoming increasingly problematic for aquatic ecologists and resource managers, as the ecological and economic impacts of introductions become better known. The flathead catfish Pylodictis olivaris is a large piscivorous fish native to most of the interior basin of the United States. Via legal and illegal introductions, they have been introduced into at least 13 U.S. states and one Canadian province primarily along the Atlantic slope. I used a variety of capture-recapture models to estimate flathead catfish population parameters in three North Carolina coastal plain rivers (Contentnea Creek, Northeast Cape Fear River, and Lumber River). My estimates using a Jolly-Seber model were hindered by low capture probabilities and high temporary emigration. Reasonable estimates were possible using a robust-design framework to estimate population size and temporary emigration with supplemental information from a radio-telemetry study to examine model assumptions. Population size estimates using a robust design model including temporary emigration ranged from 4 to 31 fish/km (>125-mm total length, TL) of sampling reach. Additional analyses showed high rates of temporary emigration (>90%), independently supported by radio-telemetry results. I also examined flathead catfish diet in these rivers and found that flathead catfish fed on a wide variety of freshwater fish and invertebrates, anadromous fish, and occasionally estuarine fish and invertebrates. Fish or crayfish comprised more than 50% of the stomach contents by percent occurrence, percent-by-number, and percent-by-weight in all rivers and years. A significant difference in the diet composition percent-by-number was found between Contentnea Creek and the Northeast Cape Fear River. Significant differences were not detected between years within Contentnea Creek but were found within the Northeast Cape Fear River. Feeding intensity (as measured by stomach fullness) was highest in the Northeast Cape Fear River associated with a lower mean size of feeding flathead catfish in this river than of those in Contentnea Creek or the Lumber River. A significant correlation between diet item length and flathead catfish total length was found for Contentnea Creek in 2001. This relationship was not significant in the Northeast Cape Fear River in either year. Based on the diet composition data collected in this study and those published on native and introduced flathead catfish populations, I am not able to support or refute the hypothesis that flathead catfish are preferentially feeding on specific species or families. However, the flathead catfish populations examined here are well established, and the greatest impact from selective predation may have occurred immediately following introduction. Based on my findings, flathead catfish could restructure or suppress native fish communities in coastal rivers through direct predation because of their primarily piscivorous food habits. To evaluate the potential ecosystem impact of this invasive species on the native fish community, I developed an ecosystem simulation model (including flathead catfish) based on empirical data collected for a North Carolina coastal river. Model results suggest that flathead catfish suppress native fish community biomass by 5-50% through both predatory and competitive interactions. However, these reductions could be mitigated through sustained exploitation of flathead catfish by recreational or commercial fishers at levels equivalent to those for native flathead catfish populations (6-25% annual exploitation). These findings demonstrate the potential for using directed harvest of an invasive species to mitigate the negative impacts to native species.

The relationships between relative standard metabolic rate, aggression, territoriality, growth and subsequent life-history strategies were studied in juvenile Atlantic salmon. In order to do this a method of calculating mass-independent relative standard metabolic rates is presented. This procedure involved using individual deviations from allometric predictions of standard metabolic rate based on body size (termed residual standard metabolic rate). As in a previous study, it was found that salmon with higher relative standard metabolic rates were more likely to acquire dominance, in both pairs and groups. However, fish with higher standard metabolic rates appeared to have smaller metabolic scopes within which they had to carry out dominance-acquiring costly activities such as aggression, although fish with higher standard metabolic rates did indeed acquire dominance through greater aggression. Fish with higher standard metabolic rates, although having a higher cost of maintenance, were found to have a lower feeding motivation, possibly because they had a smaller metabolic scope and movements associated with foraging are themselves energetically costly. Therefore it appears that juvenile salmon with high standard metabolic rates and a limited metabolic scope opt to be more aggressive and thus acquire dominance and a feeding territory at the expense of higher foraging rates, since both behavioural strategies are energetically costly. It was also found that in an environment with little food, fish with high standard metabolic rates grew less well than predicted given their position in an artificial stream than conspecifics with lower costs of maintenance. This indicates a potential cost of a high standard metabolic rate. There is a temporal component to acquisition of territories since juvenile salmon emerge from gravel redds over several days. Through this 'prior residence' effect, fish introduced into a new environment first were more likely to acquire territories than later-arriving conspecifics. First-arriving fish, as a consequence of acquiring a feeding territory, grew faster and were more likely to smolt a year earlier than late-arriving juveniles. However, they did not appear to choose the most profitable territories, implying a time constraint to searching for the best sites. If a salmon takes too long to choose a territory, it risks the territories filling up with later-arriving fish and not acquiring one at all. Prior residence appears to be a powerful asymmetry when tested in both pairs and groups, intruders having to be relatively much larger to overcome it and acquire dominance. Relative standard metabolic rate did not predict dominance when prior residence was included as a competitive asymmetry. However, fish with higher standard metabolic rates were more likely to emerge first since they absorbed their yolk-sacs faster and so needed exogenous food sooner. Therefore, a high standard metabolic rate conferred an indirect benefit since it increased the likelihood of a fish being a prior resident. Differences in aggression arising from differences in relative standard metabolic rate were also apparent in a hatchery situation. A group consisting entirely of salmon with high standard metabolic rates showed more aggression than a group of salmon with low standard metabolic rates. However, mean growth did not improve as a consequence of lower aggression rates, although the distribution of individual growth rates was more even in the group of fish with low standard metabolic rates. This may be a consequence of fewer fish in that group behaving despotically and monopolizing available food. As reported in earlier studies, differences in standard metabolic rate between the Upper and Lower Modal Groups of juvenile salmon became apparent during their first winter and spring. However, Upper Modal Group fish had higher weight-specific standard metabolic rates in December, earlier than previously documented, and higher mass-independent metabolic rates in May, prior to smoltification. This is suggested to be a pre-adaptation to the high metabolic demands the smolts will face when they migrate to sea. Individual residual standard metabolic rates varied more in the Upper Modal Group than the Lower Modal Group over winter and spring, possibly because respiratory enzymes in the Upper Modal Group were more seasonally adapted to the changing water temperatures of the period. They may have therefore worked inefficiently at the temperature at which metabolic rate was measured (since it remained constant while the ambient water temperature changed over time), being most inefficient when the difference between sampling and ambient water temperatures was greatest. However, individual residual standard metabolic rates remained broadly invariant throughout the period, demonstrating that individual standard metabolic rate is a relatively stable minimum to aerobic metabolic activity.

Data collected from the 16 species of the fish family Cyprinidae (Pisces: Teleosti) found in Britain were examined morphometrically and electrophoretically to examine the taxonomic relationships within the group and aspects of the occurrence and identification of interspecies hybrids. The analysis of morphological characters by Principal Components Analysis was not able to separate the morphological characters into clear character suites. A rooted dendrogram using Gower (1971) mixed data coefficient and UPGMA was found not to correlate strongly with the presently accepted classification or with the subsequently produced phenogram based upon the enzymatic results. The phenetic treatment of electrophoretic results gave a consensus topology equivalent to Nei's (1972) .D. and UPGMA with standard errors. Several taxonomic changes were suggested, the reduction from six to two sub-families and moving two species into different genera, Blicca bjoerkna to become Abramis bjoerkna and Alburnus alburnus to become Leuciscus leuciscus. The use of electrophoretic characters in preference to morphological ones was strongly validated. Discriminant Function Analysis could not discriminate between populations with and those without hybrids. An unidentified specimen from the River Trent, Nottingham, was also described. It was found enzymatically not to correspond to any of the 16 species examined, but could be placed in the classification in the sub-family Leuciscinae and Abramin lineage.