William J. Lindberg

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.

Influence of landscape structure on reef fish assemblages

Management of tropical marine environments calls for interdisciplinary studies and innovative methodologies that consider processes occurring over broad spatial scales. We investigated relationships between landscape structure and reef fish assemblage structure in the US Virgin Islands. Measures of landscape structure were transformed into a reduced set of composite indices using principal component analyses (PCA) to synthesize data on the spatial patterning of the landscape structure of the study reefs. However, composite indices (e.g., habitat diversity) were not particularly informative for predicting reef fish assemblage structure. Rather, relationships were interpreted more easily when functional groups of fishes were related to individual habitat features. In particular, multiple reef fish parameters were strongly associated with reef context. Fishes responded to benthic habitat structure at multiple spatial scales, with various groups of fishes each correlated to a unique suite of variables. Accordingly, future experiments should be designed to test functional relationships based on the ecology of the organisms of interest. Our study demonstrates that landscape-scale habitat features influence reef fish communities, illustrating promise in applying a landscape ecology approach to better understand factors that structure coral reef ecosystems. Furthermore, our findings may prove useful in design of spatially-based conservation approaches such as marine protected areas (MPAs), because landscape-scale metrics may serve as proxies for areas with high species diversity and abundance within the coral reef landscape.

Comparative patterns of occupancy by decapod crustaceans in seagrass, oyster, and marsh-edge habitats in a Northeast Gulf of Mexico estuary

Decapod crustaceans occupying seagrass, salt marsh edge, and oyster habitats within the St. Martins Aquatic Preserve along the central Gulf coast of Florida were quantitatively sampled using a 1-m2 throw trap during July–August 1999 and March–April 2000. Relative abundance and biomass were used as the primary measures to compare patterns of occupancy among the three habitat types. Representative assemblages of abundant and common species from each habitat were compared using Schoeners Percent Similarity Index (PSI). In all, 17,985 decapods were sampled, representing 14 families and 28 species. In the summer sampling period, mean decapod density did not differ between oyster and seagrass habitats, which both held greater densities of decapods than marsh-edge. In the spring sampling period oyster reef habitat supported greater mean decapod density than both seagrass and marsh-edge, which had similar densities of decapods. Habitat-specific comparisons of decapod density between the two sampling periods indicated no clear seasonal effect. In summer 1999, when seagrasses were well established, decapod biomass among the three habitats was not significantly different. During spring 2000, decapod biomass in oyster (41.40 gm−2) was greater than in marshedge (4.20 gm−2), but did not differ from that of seagrass (9.73 g m−2). There was no significant difference in decapod biomas between seagrass and marsh-edge habitats during the spring 2000 sampling period. The assemblage analysis using Schoeners PSI indicated that decapod assemblages associated with oyster were distinct from seagrass and marshedge habitats (which were similar). The results of this study suggest that in comparison to seagrass and marsh-edge habitats, oyster reef habitats and the distinct assemblage of decapod crustaceans that they support represent an ecologically important component of this estuarine system.

Artificial Reefs, the Attraction-production Issue, and Density Dependence in Marine Ornamental Fishes

Artificial reefs may provide a useful tool to enhance production of marine ornamentals and to divert detrimental harvesting activities from sensitive natural habitat. The efficacy of this strategy depends, in part, on the extent to which artificial reefs contribute to new production (vs. attract fishes from natural habitat) and therefore benefit harvested populations on a local and regional basis. Here the attraction and production hypotheses and their application to marine ornamentals are presented. Specifically, it is discussed how the strength and timing of density dependence can affect the response of fish population dynamics to artificial reefs. In addition to this discussion, examples of density dependence in marine ornamentals and related reef fishes are provided. Based on this information, a simple conceptual model is presented to clarify the role of density dependence, and this is followed by a discussion on the use of artificial reefs in the management and production of marine ornamentals. Finally, unresolved scientific issues that remain to be addressed are provided.

The effects of temperature and salinity on survival and development of early life stage Florida stone crabs Menippe mercenaria (Say)

Abstract The effects of temperature and salinity on survival and growth of early life stage Menippe mercenaria (Say) were tested by rearing larvae and small juveniles (⩽10 mm CW) in factorial temperature-salinity arrays (larvae: 20–35°C [5°C increments], 10–40ℵ [10t% increments]; juveniles: 5–35°C [5°C increments], 10–40ℵ [5ℵ increments]). The highest proportion of larvae survived in water of 30°C, 30ℵ; juvenile survival was 100% in water ranging from 15 to 25°C and from 25 to 40ℵ. The distribution of this species both latitudinally and seaward may be determined by temperature and salinity tolerance limits of early stages. Both temperature and salinity affected the survival of early zoeal stages (Z1–3). Salinity effects decreased during late zoeal stages (Z4–Z5) and then increased in the megalopal and juvenile stages. The decrease in the effect of salinity on survival at zoeal stage four suggests that osmoregulatory ability begins at this stage. The occurence of a supernumerary zoeal stage (Z6) was not influenced by temperature or salinity and did not affect survival. Both larval developmental rates and frequency of molting in juveniles accelerated with increasing temperature but were not influenced by salinity. The observed effects of temperature on molting would promote rapid growth when water temperatues are warm, but would delay this physiologically stressful event when temperatures are cool and marginal for survival. The lower optimal temperature for survival of juveniles is probably an adaptation brought about by timing of the major reproductive season.

Training Grass Carp to Respond to Sound for Potential Lake Management Uses

Abstract Future use of triploid grass carp Ctenopharyngodon idella as a control agent for aquatic macrophytes could be enhanced by efficient methods of removing the fish from stocked waters. In laboratory trials, we evaluated the ability to train triploid grass carp to move to a specific area for potential recapture by coupling low-frequency sounds (i.e., 1,000, 800, and 600 Hz) and grass carp feeding sounds with a food reward. Repeated-measures analysis of variance (ANOVA) indicated that grass carp response times to sound stimuli declined over training sessions for all low-frequency sounds tested. Percent return rates averaged 94, 94, 88, and 71% for low-frequency sounds of 600, 800, and 1,000 Hz and grass carp feeding sounds, respectively. Our results suggest that sound can be used to attract triploid grass carp to a central area where they potentially can be removed.