Andrew L. Rypel

Growth and longevity in freshwater mussels: evolutionary and conservation implications

The amount of energy allocated to growth versus other functions is a fundamental feature of an organisms life history. Constraints on energy availability result in characteristic trade‐offs among life‐history traits and reflect strategies by which organisms adapt to their environments. Freshwater mussels are a diverse and imperiled component of aquatic ecosystems but little is known about their growth and longevity. Generalized depictions of freshwater mussels as ‘long‐lived and slow‐growing’ may give an unrealistically narrow view of life‐history diversity which is incongruent with the taxonomic diversity of the group and can result in development of inappropriate conservation strategies. We investigated relationships among growth, longevity, and size in 57 species and 146 populations of freshwater mussels using original data and literature sources. In contrast to generalized depictions, longevity spanned nearly two orders of magnitude, ranging from 4 to 190 years, and the von Bertalanffy growth constant, K, spanned a similar range (0.02–1.01). Median longevity and K differed among phylogenetic groups but groups overlapped widely in these traits. Longevity, K, and size also varied among populations; in some cases, longevity and K differed between populations by a factor of two or more. Growth differed between sexes in some species and males typically reached larger sizes than females. In addition, a population of Quadrula asperata exhibited two distinctly different growth trajectories. Most individuals in this population had a low‐to‐moderate value of K (0.15) and intermediate longevity (27 years) but other individuals showed extremely slow growth (K = 0.05) and reached advanced ages (72 years). Overall, longevity was related negatively to the growth rate, K, and K explained a high percentage of variation in longevity. By contrast, size and relative shell mass (g mm−1 shell length) explained little variation in longevity. These patterns remained when data were corrected for phylogenetic relationships among species. Path analysis supported the conclusion that K was the most important factor influencing longevity both directly and indirectly through its effect on shell mass. The great variability in age and growth among and within species shows that allocation to growth is highly plastic in freshwater mussels. The strong negative relationship between growth and longevity suggests this is an important trade‐off describing widely divergent life‐history strategies. Although life‐history strategies may be constrained somewhat by phylogeny, plasticity in growth among populations indicates that growth characteristics cannot be generalized within a species and management and conservation efforts should be based on data specific to a population of interest.

Simple ecological trade-offs give rise to emergent cross-ecosystem distributions of a coral reef fish

Ecosystems are intricately linked by the flow of organisms across their boundaries, and such connectivity can be essential to the structure and function of the linked ecosystems. For example, many coral reef fish populations are maintained by the movement of individuals from spatially segregated juvenile habitats (i.e., nurseries, such as mangroves and seagrass beds) to areas preferred by adults. It is presumed that nursery habitats provide for faster growth (higher food availability) and/or low predation risk for juveniles, but empirical data supporting this hypothesis is surprisingly lacking for coral reef fishes. Here, we investigate potential mechanisms (growth, predation risk, and reproductive investment) that give rise to the distribution patterns of a common Caribbean reef fish species, Haemulon flavolineatum (French grunt). Adults were primarily found on coral reefs, whereas juvenile fish only occurred in non-reef habitats. Contrary to our initial expectations, analysis of length-at-age revealed that growth rates were highest on coral reefs and not within nursery habitats. Survival rates in tethering trials were 0% for small juvenile fish transplanted to coral reefs and 24–47% in the nurseries. As fish grew, survival rates on coral reefs approached those in non-reef habitats (56 vs. 77–100%, respectively). As such, predation seems to be the primary factor driving across-ecosystem distributions of this fish, and thus the primary reason why mangrove and seagrass habitats function as nursery habitat. Identifying the mechanisms that lead to such distributions is critical to develop appropriate conservation initiatives, identify essential fish habitat, and predict impacts associated with environmental change.

Growth of Freshwater Drum from Lotic and Lentic Habitats in Alabama

Abstract We analyzed variations in the age and growth of freshwater drum Aplodinotus grunniens in Alabama rivers and reservoirs. Lotic environments produced significantly more robust freshwater drum (associated with higher growth rates) than did lentic settings. Shorter reservoir retention times (more lotic reservoirs) also produced more robust and faster-growing freshwater drum, which supported the hypothesis that hydraulic-based habitat was an important variable in determining freshwater drum condition and growth. Linear declines in relative weight and percent lipid were revealed along a hydrologic gradient in Lake Logan Martin and were strongest in older freshwater drum. Von Bertalanffy predictions supported these results but also exposed a unique caveat: If freshwater drum lived long enough (>12 years), they were ultimately able to grow to larger sizes (>L ∞) in reservoirs. Freshwater drum may be capable of moving between available lentic and lotic habitats during their lifetime to maximize invertebra...

Mercury Concentrations in Lentic Fish Populations Related to Ecosystem and Watershed Characteristics

Predicting mercury (Hg) concentrations of fishes at large spatial scales is a fundamental environmental challenge with the potential to improve human health. In this study, mercury concentrations were examined for five species across 161 lakes and ecosystem, and watershed parameters were investigated as explanatory variables in statistical models. For all species, Hg concentrations were significantly, positively related to wetland coverage. For three species (largemouth bass, pike, and walleye), Hg concentrations were significantly, negatively related to lake trophic state index (TSI), suggestive of growth biodilution. There were no significant relationships between ecosystem size and mercury concentrations. However, Hg concentrations were strongly, positively related to ecosystem size across species. Scores of small or remote lakes that have never been tested could be prioritized for testing using models akin to those presented in this article. Such an approach could also be useful for exploring how Hg concentrations of fishes might respond to natural or anthropogenic changes to ecosystems over time.

Inland Fisheries Habitat Management: Lessons Learned from Wildlife Ecology and a Proposal for Change

The habitat concept in inland fisheries has been less studied than wildlife ecology. Since 1950, the cumulative number of publications about “freshwater or inland habitat and fisheries management” has been 60%–95% less than those considering “habitat and wildlife management.” The number of publications about “marine, river, and stream habitat and fisheries management” has also generally exceeded those for “lake habitat and fisheries management.” We provide a perspective comparing inland fish and wildlife habitat management systems and highlight lessons from wildlife ecology that could benefit inland fisheries. We reason that wildlife habitat management has become widespread and accepted because humans share habitats with wildlife and positive/negative responses to habitat restorations/loss are directly observable. We recommend that inland fisheries habitat studies and restorations include opportunities for humans to directly observe the ecological benefits of such practices. To support aquatic habitat man...

Do fish growth rates correlate with PCB body burdens

We evaluated whether growth rates of six fish species correlated with PCB concentrations in a moderately-to-heavily polluted freshwater ecosystem. Using a large dataset (n = 984 individuals), and after accounting for growth effects related to fish age, habitat, sex, and lipids, growth correlated significantly, but positively with lipid-corrected PCB concentrations for 4 of 6 species. Remaining species showed no correlations between growth and PCBs. Comparisons with regional, lentic growth averages for four species confirmed growth was on par and in three of four cases higher than regional averages in the PCB-polluted ecosystem. We conclude that for these species, at the range of concentrations examined, these PCBs do not exert negative impacts on growth. Rather, factors often cited as influential to growth were also driving growth trends in this study. Future studies that evaluate whether pollution affects growth must account for major growth drivers prior to attributing growth differentials to pollution alone.

Summer Nocturnal Patterns in Freshwater Drum (Aplodinotus grunniens)

ABSTRACT We observed diel patterns in CPUE, size and age-structure for freshwater drum (Aplodinotus grunniens) from Alabama waters during May–July 2003 . Drum were significantly more susceptible to capture along shoreline habitat at night. CPUE of freshwater drum was often zero during the day and increased up to 84.5 fish * h−1 at night suggesting that many drum shift location to nearshore habitat after dark. Drum captured at night were also significantly smaller and younger that drum captured during the day. Age-0 drum were the most abundant age-class in nighttime samples whereas age-8 drum were the most abundant age-class in daytime samples. The dynamic interplay between food availability and predation on juvenile fish likely drives diel habitat shifts of freshwater drum.

Water Body Type Influences Climate–Growth Relationships of Freshwater Drum

Abstract Climate–growth relationships of fish are of increasing research interest. However, few studies have attempted to characterize the extent to which climate–growth relationships may diverge across fundamentally different types of environments. The goal of this study was to examine the effects of climate variability (i.e., temperature, precipitation, global climate indices) on the growth of Freshwater Drum Aplodinotus grunniens and determine the extent to which these relationships differ between lotic and lentic habitats. Freshwater Drum were collected from five river and five reservoir environments in Georgia, Alabama, and Mississippi. Tree-ring techniques were used to standardize annual growth for age (measured using interannual growth increments from otolith sagittae) and growth indices were correlated to annual thermal, hydrologic, and global climate indices. Freshwater Drum expressed distinct and significant climate–growth relationships by water body type. The general pattern observed was that a...