Debra J. Murie

Density-dependent habitat selection and performance by a large mobile reef fish.

Many exploited reef fish are vulnerable to overfishing because they concentrate over hard-bottom patchy habitats. How mobile reef fish use patchy habitat, and the potential consequences on demographic parameters, must be known for spatially explicit population dynamics modeling, for discriminating essential fish habitat (EFH), and for effectively planning conservation measures (e.g., marine protected areas, stock enhancement, and artificial reefs). Gag, Mycteroperca microlepis, is an ecologically and economically important warm-temperate grouper in the southeastern United States, with behavioral and life history traits conducive to large-scale field experiments. The Suwannee Regional Reef System (SRRS) was built of standard habitat units (SHUs) in 1991-1993 to manipulate and control habitat patchiness and intrinsic habitat quality, and thereby test predictions from habitat selection theory. Colonization of the SRRS by gag over the first six years showed significant interactions of SHU size, spacing, and reef age; with trajectories modeled using a quadratic function for closely spaced SHUs (25 m) and a linear model for widely spaced SHUs (225 m), with larger SHUs (16 standardized cubes) accumulating significantly more gag faster than smaller 4-cube SHUs (mean = 72.5 gag/16-cube SHU at 225-m spacing by year 6, compared to 24.2 gag/4-cube SHU for same spacing and reef age). Residency times (mean = 9.8 mo), indicative of choice and measured by ultrasonic telemetry (1995-1998), showed significant interaction of SHU size and spacing consistent with colonization trajectories. Average relative weight (W(r)) and incremental growth were greater on smaller than larger SHUs (mean W(r) = 104.2 vs. 97.7; incremental growth differed by 15%), contrary to patterns of abundance and residency. Experimental manipulation of shelter on a subset of SRRS sites (2000-2001) confirmed our hypothesis that shelter limits local densities of gag, which, in turn, regulates their growth and condition. Density-dependent habitat selection for shelter and individual growth dynamics were therefore interdependent ecological processes that help to explain how patchy reef habitat sustains gag production. Moreover, gag selected shelter at the expense of maximizing their growth. Thus, mobile reef fishes could experience density-dependent effects on growth, survival, and/or reproduction (i.e., demographic parameters) despite reduced stock sizes as a consequence of fishing.

Comparative age and growth of Nile tilapia (Oreochromis niloticus L.) in lakes Nabugabo and Wamala, Uganda

Age and growth of Nile tilapia (Oreochromis niloticus) from Lake Nabugabo and Lake Wamala, Uganda, were determined using cross-sectioned sagittal otoliths. Marginal-increment and edge analyses of Nile tilapia otoliths from Lake Nabugabo indicated formation of two annuli per 12-month period. Opaque zones associated with faster growth were observed between April and June and between September and December, coincident with the two rainy seasons of the year. Within both lakes, males were larger at age than females. Nile tilapia from Lake Nabugabo, however, had faster growth rates than Nile tilapia from Lake Wamala, and fish >3 years old from Lake Nabugabo were larger at age than those from Lake Wamala. Ages ranged from 0 to 8.0 years for Nile tilapia from Lake Nabugabo, and from 0.5 to 6.5 years for tilapia from Lake Wamala. Differences in the patterns of growth in Nile tilapia between lakes may reflect, at least in part, the relatively energy-rich omnivorous diet of Nile tilapia in Lake Nabugabo versus a phytoplanktivorous diet in Lake Wamala. Diet differences of Nile tilapia between the two lakes are ascribed to trophic changes in the lakes due to the introduction of Nile perch (Lates niloticus) into Lake Nabugabo but not Lake Wamala. Alternatively, the greater exploitation of Nile tilapia in Lake Nabugabo may have resulted in increased growth rates, whereas Nile tilapia in Lake Wamala may be subject to slower, density-dependent growth.

Distribution of Gulf of Mexico Sturgeon in Relation to Benthic Invertebrate Prey Resources and Environmental Parameters in the Suwannee River Estuary, Florida

Abstract The distribution of threatened Gulf of Mexico sturgeon Acipenser oxyrinchus desotoi (hereafter referred to as Gulf sturgeon) in the Suwannee River estuary, Florida, was examined relative to the distribution of benthic invertebrate prey and environmental variables (salinity, temperature, dissolved oxygen, and sediment type) to determine potential foraging areas within the estuary. Eighteen Gulf sturgeon (1,279–2,010 mm total length, 15.25–53.25 kg) were netted at the mouth of the Suwannee River during their spring upriver migration in 2001 and surgically implanted with ultrasonic tags. These sturgeon were subsequently tracked as they migrated seaward out of the Suwannee River and into the estuary in fall 2001 and as they returned to the estuary in spring 2002 to migrate upriver again. Invertebrates and sediment type were identified in diver-collected cores from the Suwannee River estuary during spring 2002. Thirteen of the 18 tagged Gulf sturgeon were relocated in the Suwannee River estuary. Eight...

Augmentation of French grunt diet description using combined visual and DNA-based analyses

Trophic linkages within a coral-reef ecosystem may be difficult to discern in fish species that reside on, but do not forage on, coral reefs. Furthermore, dietary analysis of fish can be difficult in situations where prey is thoroughly macerated, resulting in many visually unrecognisable food items. The present study examined whether the inclusion of a DNA-based method could improve the identification of prey consumed by French grunt, Haemulon flavolineatum, a reef fish that possesses pharyngeal teeth and forages on soft-bodied prey items. Visual analysis indicated that crustaceans were most abundant numerically (38.9%), followed by sipunculans (31.0%) and polychaete worms (5.2%), with a substantial number of unidentified prey (12.7%). For the subset of prey with both visual and molecular data, there was a marked reduction in the number of unidentified sipunculans (visual – 31.1%, combined – 4.4%), unidentified crustaceans (visual – 15.6%, combined – 6.7%), and unidentified taxa (visual – 11.1%, combined – 0.0%). Utilising results from both methodologies resulted in an increased number of prey placed at the family level (visual – 6, combined – 33) and species level (visual – 0, combined – 4). Although more costly than visual analysis alone, our study demonstrated the feasibility of DNA-based identification of visually unidentifiable prey in the stomach contents of fish.

Effects of Variable Flows on Water Chemistry Gradients and Fish Communities at the Hillsborough River, Florida

Abstract We evaluated the effects of variable flows on water chemistry and fish communities to recommend biologically based minimum flows that protect low-salinity zones for freshwater–oligohaline (FO) fishes below the Hillsborough River Dam, Tampa, Florida. We plotted the distributions of hypoxic (dissolved oxygen (DO) ≤ 4 mg/L) and meso- and polyhaline (salinity > 5‰) habitats and evaluated changes in species richness and spatial distributions of FO fishes on five sampling dates from October 2002 to July 2004. During low flows (0.12 m3/s; 5‰) migrated upstream to within 1,300 m of the dam and DO decreased to less than 4 mg/L in the lower part of the reach. However, intermediate (1.19–2.18 m3/s; 25th–50th percentiles) and high flows (5.49 m3/s; 50th–75th percentiles) on four of the five sampling dates maintained oligohaline salinities (i.e., ≤5‰) and DO greater than 4 mg/L throughout most of the reach. Using fyke net data, w...

Postrelease Rate of Loss of Juvenile Red Drum Stocked out of Season in the Chassahowitzka National Wildlife Refuge, Florida

Abstract Rates of loss of juvenile red drum Sciaenops ocellatus (∼85 mm total length) stocked out of season were estimated using hatchery releases and ultrasonically tagged fish in Pumpkin and May creeks in the Chassahowitzka National Wildlife Refuge on the Gulf coast of Florida. In Pumpkin Creek, approximately 7,000 of the 10,000 fish released on 27 June 2000 survived combined transport mortality (2%) and 24-h acute mortality (28 ± 8.7% [mean ± SE] n = 6). In May Creek, approximately 5,100 of the 10,000 fish released on 29 June 2000 survived combined transport (7.5–10%) and 24-h acute (44 ± 0%, n = 2) mortality. Catch curve estimates of the instantaneous “rate of loss” (mortality or emigration) within 5 d were 0.46/d and 0.50/d for Pumpkin and May creeks, respectively. Red drum with ultrasonic transmitters (n = 12) also rapidly “disappeared” from the creeks within 4 d after release. These are the first estimates of postrelease rate of loss for juvenile red drum released out of season on the west coast of...

Fish condition in introduced tilapias of Ugandan crater lakes in relation to deforestation and fishing pressure

This study identifies environmental predictors of the condition of two introduced tilapia species (Oreochromis leucostictus and Tilapia zillii) that are known to have divergent trophic niches (planktivore and herbivore, respectively) in 17 crater lakes in western Uganda. We asked whether fish condition differs among lakes characterized by differences in fishing pressure and catchment deforestation; and we related relative condition factor to gradients of environmental variation across lakes. Lakes characterized by severe catchment deforestation tended to be lakes with high fishing pressure, so it was difficult to explore independent and interactive effects. However, mean relative condition factor was higher in populations with high fishing pressure compared to populations with low fishing pressure for both O. leucostictus and T. zillii. The condition of O. leucostictus populations was higher in lakes with severely deforested catchments; but mean relative condition factor of T. zillii did not differ between deforestation categories. Principal components analysis (PCA) was used to describe the major environmental gradients of variation among the lakes; and PCA factor scores were regressed against relative fish condition. The association between fish condition and environmental gradients was stronger for O. leucostictus than for T. zillii. For O. leucostictus, fish condition was related to PC1 (43% of the variance) and factors that loaded most heavily included Chl-a, water transparency, lake area and depth, suggesting higher condition in lakes characterized by higher primary productivity and smaller size. For T. zillii, PC3 (11%) was the only axis related to fish condition; and factors that loaded most heavily included lake area (positive), and conductivity and total nitrogen (negative). Some of the larger lakes are characterized by higher availability of macrophytes that may positively affect the food base for T. zillii.

Optimum Dietary Protein Level for Growth and Protein Efficiency without Hepatocyte Changes in Juvenile African Cichlids Pseudotropheus socolofi

Abstract Six diets were formulated with protein levels ranging from 32.5% to 58.8% to determine the dietary protein level that would provide adequate growth and enhanced protein efficiency without pathological changes in the hepatocytes of juvenile ornamental African cichlids Pseudotropheus socolofi. Fish (0.11 g; 1.8 cm) were fed three times daily at 12% body weight per day for 10 weeks. Survival was greater than 95% for all treatments. At the end of the trial, fish on all diets had similar final lengths (P = 0.21). Final weight and final weight gain were significantly different among treatments (P = 0.04 and 0.03, respectively), and both variables exhibited a graded response. Final weight and weight gain increased as the protein level increased to 42.5% and then leveled off at protein levels greater than 42.5%. For fish on all diets, the final carcass protein composition was similar (P > 0.05) to the initial composition. Protein efficiency ratio and protein conversion efficiency decreased with an increa...

Nonlethal Sex Determination of the Greater Amberjack, with Direct Application to Sex Ratio Analysis of the Gulf of Mexico Stock

Abstract Stock assessments for the Gulf of Mexico (Gulf) Greater Amberjack Seriola dumerili continue to designate the stock as overfished and undergoing overfishing, despite increasing regulatory measures. Knowledge of sex-specific spatial distribution and fishing mortality may contribute to our understanding of the stocks overexploitation, especially since Greater Amberjacks may be subject to sex-specific mortality resulting from minimum size regulations. Currently, the sex ratio of the stock is assumed to be 1:1. An average and range of sex ratios were estimated for the Gulf stock based on sampling of fish landed in the recreational and commercial fisheries and based on released fish that were nonlethally sexed during a tagging study of sex-specific movement patterns and release mortality. The nonlethal method of sex determination was developed based on external features of the urogenital region; urogenital catheterization was used to validate the external sexing and to collect oocyte samples for determining maturity stages of females during the spawning season. Of the 238 fish (108 males and 130 females) for which sex was verified, only one smaller female was incorrectly sexed. Urogenital catheterization identified females that were spawning or that would likely spawn in the upcoming spawning season, but no differentiation could be made between immature and resting females. Analysis of published data sets suggested that the Gulf stock has an overall female-skewed sex ratio, with estimates ranging from 1:1.7 to 1:2.5 (male : female), while the nonlethal sexing data from the current study suggest that the overall sex ratio is slightly male skewed (1:0.8) in some regions. All studies report a female-skewed sex ratio of 1:2.3, on average, for 1,000-mm FL and larger fish in the Gulf. Sex ratios that deviate from the assumed 1:1 ratio should be incorporated into future assessments of the Gulf Greater Amberjack stock to investigate potential consequences for stock status and management.

An Evaluation of Two Commercially Prepared Feeds on Growth Performance and Liver Condition of Juvenile African Cichlids Pseudotropheus socolofi and Haplochromis ahli

Abstract A 12-week feeding trial was conducted to evaluate growth performance and hepatocyte changes in juvenile African cichlids Pseudotropheus socolofi and Haplochromis ahli fed commercial diets commonly used on cichlid farms in south Florida. Fish were fed either a trout starter pellet diet (TP diet; 52% crude protein, 17% lipid) or a mixed flake feed diet (fish flake [FF] diet) (47% crude protein, 7% lipid). For both species, growth was significantly greater (P 0.1). Histological examination revealed severe vacuolation of hepatocytes in P. socolofi fed the TP diet and moderate vacuolation in H. ahli fed the TP diet. Moderate vacuolation of hepatocytes was observed in P. socolofi fed the FF diet, and only mild vacuolation was found in H. ahli fed the FF diet. The lipid-rich TP diet may be suitable for ...