Nathan M. Bacheler

A combined telemetry - tag return approach to estimate fishing and natural mortality rates of an estuarine fish

A joint analysis of tag return and telemetry data should improve estimates of mortality rates for exploited fishes; however, the combined approach has thus far only been tested in terrestrial systems. We tagged subadult red drum (Sciae- nops ocellatus) with conventional tags and ultrasonic transmitters over 3 years in coastal North Carolina, USA, to test the efficacy of the combined telemetry - tag return approach. There was a strong seasonal pattern to monthly fishing mortality rate (F) estimates from both conventional and telemetry tags; highest F values occurred in fall months and lowest levels occurred during winter. Although monthly F values were similar in pattern and magnitude between conventional tagging and telemetry, information on F in the combined model came primarily from conventional tags. The estimated natural mortality rate (M) in the combined model was low (estimated annual rate ± standard error: 0.04 ± 0.04) and was based pri- marily upon the telemetry approach. Using high-reward tagging, we estimated different tag reporting rates for state agency and university tagging programs. The combined telemetry - tag return approach can be an effective approach for estimat- ing F and M as long as several key assumptions of the model are met.

Ontogenetic and sex‐specific differences in density‐dependent habitat selection of a marine fish population

The spatial dynamics of species are the result of complex interactions between density-independent and density-dependent sources of variability. Disentangling these two sources of variability has challenged ecologists working in both terrestrial and aquatic ecosystems. Using a novel spatially explicit statistical model, we tested for the presence of density-independent and density-dependent habitat selection in yellowfin sole (Limanda aspera) in the eastern Bering Sea. We found specificities in the density-dependent processes operating across ontogeny and particularly with gender. Density-dependent habitat expansion occurred primarily in females, and to a lesser degree in males. These patterns were especially evident in adult stages, while juvenile stages of both sexes exhibited a mix of different dynamics. Association of yellowfin sole with substrate type also varied by sex and to a lesser degree with size, with large females distributed over a wider range of substrates than males. Moreover, yellowfin sole expanded northward as cold subsurface waters retracted in summer, suggesting high sensitivity to arctic warming. Our findings illustrate how marginal habitats can play an important role in buffering density-dependent habitat expansion, with direct implications for resource management. Our spatially explicit modeling approach is effective in evaluating density-dependent spatial dynamics, and can easily be used to test similar hypotheses from a variety of aquatic and terrestrial ecosystems.

Movements and Mortality Rates of Striped Mullet in North Carolina

Abstract Striped mullet Mugil cephalus is an economically important species to commercial and recreational fishermen, as well as an ecologically significant detritivore linking lower trophic levels with a wide variety of estuarine and marine fish and birds. Despite this importance, striped mullet migration patterns and mortality rates are poorly understood. Approximately 15,000 striped mullet were tagged in North Carolina between 1997 and 2001, and monthly movement information was collected on recovered individuals (n = 384) from commercial and recreational fishermen and state agency personnel. A tag return model was used to estimate an instantaneous total mortality rate, and this rate was partitioned into natural and fishing components by means of life history methods. Nearly all (98.2%) striped mullet were recovered in North Carolina, the remaining few being recovered in nearby states. Striped mullet moved southward of their tagging locations and had the highest daily movement rate between the months of...

Recruitment of Juvenile Red Drum in North Carolina: Spatiotemporal Patterns of Year‐Class Strength and Validation of a Seine Survey

Abstract Sixteen years of data from a North Carolina Division of Marine Fisheries seining survey were analyzed to assess the validity of the survey, describe spatial and temporal patterns in recruitment of red drum Sciaenops ocellatus, and evaluate factors that potentially contribute to recruitment variation. Overall, 11,817 age-0 red drum (total length = 11–104 mm) were collected in the survey during 1991–2006. Annual estimates of red drum recruitment were variable over time and showed no long-term trend. The dome-shaped catch-per-unit-effort (CPUE) pattern of all annual cohorts indicated that the seine survey captured peak abundance levels each year; the spatial coverage of the survey also appeared to be adequate. Regional correlations within North Carolina were strongest for areas that were adjacent to one another, and the North Carolina statewide index was not correlated with indices from Florida or Texas, suggesting that factors controlling recruitment of red drum operate at a scale of tens to hundre...

Occupancy Models for Monitoring Marine Fish: A Bayesian Hierarchical Approach to Model Imperfect Detection with a Novel Gear Combination

Occupancy models using incidence data collected repeatedly at sites across the range of a population are increasingly employed to infer patterns and processes influencing population distribution and dynamics. While such work is common in terrestrial systems, fewer examples exist in marine applications. This disparity likely exists because the replicate samples required by these models to account for imperfect detection are often impractical to obtain when surveying aquatic organisms, particularly fishes. We employ simultaneous sampling using fish traps and novel underwater camera observations to generate the requisite replicate samples for occupancy models of red snapper, a reef fish species. Since the replicate samples are collected simultaneously by multiple sampling devices, many typical problems encountered when obtaining replicate observations are avoided. Our results suggest that augmenting traditional fish trap sampling with camera observations not only doubled the probability of detecting red snapper in reef habitats off the Southeast coast of the United States, but supplied the necessary observations to infer factors influencing population distribution and abundance while accounting for imperfect detection. We found that detection probabilities tended to be higher for camera traps than traditional fish traps. Furthermore, camera trap detections were influenced by the current direction and turbidity of the water, indicating that collecting data on these variables is important for future monitoring. These models indicate that the distribution and abundance of this species is more heavily influenced by latitude and depth than by micro-scale reef characteristics lending credence to previous characterizations of red snapper as a reef habitat generalist. This study demonstrates the utility of simultaneous sampling devices, including camera traps, in aquatic environments to inform occupancy models and account for imperfect detection when describing factors influencing fish population distribution and dynamics.

An Age-Dependent Tag Return Model for Estimating Mortality and Selectivity of an Estuarine-Dependent Fish with High Rates of Catch and Release

Abstract Red drum Sciaenops ocellatus support commercial and recreational fisheries in North Carolina, but the stock was overfished in the 1980s because fishing was unregulated. Subsequent fishery regulations increased subadult survival into adult age-classes, but overall stock status is difficult to assess because of migration to ocean waters, prohibited harvest of older fish, and relative importance of catch and release. We analyzed 24 years of tagging data from the North Carolina Division of Marine Fisheries to assess the effects of two regulation changes (effected in 1991 and 1998) on fishing mortality rate (F) and selectivity (SEL) patterns of red drum. We used an age-dependent tag return model that accounted for both harvest and catch-and-release fishing. Using external estimates of natural mortality (M) and annual tag retention rate, we obtained precise estimates of annual F; an overall tag reporting rate (λ); and fate-specific, age-specific, and regulation-period-specific SEL. Estimated F of fully...

Invasive lionfish reduce native fish abundance on a regional scale

Invasive lionfish pose an unprecedented threat to biodiversity and fisheries throughout Atlantic waters off of the southeastern United States, the Caribbean, and the Gulf of Mexico. Here, we employ a spatially replicated Before-After-Control-Impact analysis with temporal pairing to quantify for the first time the impact of the lionfish invasion on native fish abundance across a broad regional scale and over the entire duration of the lionfish invasion (1990–2014). Our results suggest that 1) lionfish-impacted areas off of the southeastern United States are most prevalent off-shore near the continental shelf-break but are also common near-shore and 2) in impacted areas, lionfish have reduced tomtate (a native forage fish) abundance by 45% since the invasion began. Tomtate served as a model native fish species in our analysis, and as such, it is likely that the lionfish invasion has had similar impacts on other species, some of which may be of economic importance. Barring the development of a control strategy that reverses the lionfish invasion, the abundance of lionfish in the Atlantic, Caribbean, and Gulf of Mexico will likely remain at or above current levels. Consequently, the effect of lionfish on native fish abundance will likely continue for the foreseeable future.

Influence of soak time and fish accumulation on catches of reef fishes in a multispecies trap survey

Catch rates from fishery-independent surveys often are assumed to vary in proportion to the actual abundance of a population, but this approach assumes that the catchability coefficient (q) is constant. When fish accumulate in a gear, the rate at which the gear catches fish can decline, and, as a result, catch asymptotes and q declines with longer fishing times. We used data from long-term trap surveys (1990–2011) in the southeastern U.S. Atlantic to determine whether traps saturated for 8 reef fish species because of the amount of time traps soaked or the level of fish accumulation (the total number of individuals of all fish species caught in a trap). We used a delta-generalized-additive model to relate the catch of each species to a variety of predictor variables to determine how catch was influenced by soak time and fish accumulation after accounting for variability in catch due to the other predictor variables in the model. We found evidence of trap saturation for all 8 reef fish species examined. Traps became saturated for most species across the range of soak times examined, but trap saturation occurred for 3 fish species because of fish accumulation levels in the trap. Our results indicate that, to infer relative abundance levels from catch data, future studies should standardize catch or catch rates with nonlinear regression models that incorporate soak time, fish accumulation, and any other predictor variable that may ultimately influence catch. Determination of the exact mechanisms that cause trap saturation is a critical need for accurate stock assessment, and our results indicate that these mechanisms may vary considerably among species.

Depth-Related Distribution of Postjuvenile Red Snapper in Southeastern U.S. Atlantic Ocean Waters: Ontogenic Patterns and Implications for Management

Abstract For the economically and ecologically important Red Snapper Lutjanus campechanus, depth distribution patterns across ontogeny are not well understood, particularly in the southeastern U.S. Atlantic Ocean (SEUSA). Using data derived from two fishery-independent surveys targeting hardbottom habitats, we examined patterns of age- and length-specific depth distributions of postjuvenile (age 1+) Red Snapper in the SEUSA. We also compared age and length distributions between fishery-independent surveys and commercial hook-and-line catches to make inferences about gear-specific age and size selectivity, which could have implications for gear-specific interpretations of Red Snapper depth distribution patterns and for determining selectivity functions used in stock assessments. Older, larger Red Snapper were generally distributed throughout all depths, whereas the younger and smaller Red Snapper occurred disproportionately in relatively shallow waters. For Red Snapper equal to or larger than 50 cm FL, we found no evidence of a positive relationship between depth and age or length. Additionally, age and length distributions of Red Snapper ≥ 50 cm FL did not differ between fishery-independent surveys and the commercial hook-and-line fishery. These results provide no support for assertions of greater abundances of older and larger Red Snapper in deeper SEUSA waters. As observed in this study for Red Snapper in SEUSA waters, we suggest that patterns of increasing age and size with depth for multiple reef-associated fish species in SEUSA and Gulf of Mexico waters may be driven by younger and smaller fish occurring in shallower waters, and older and larger fish being distributed more equally across depths. Analyses to test this hypothesis for multiple species would be informative for their assessment and management and are recommended.

Controls on Abundance and Distribution of Yellow Perch: Predator, Water Quality, and Density-Dependent Effects

Abstract Unraveling the diverse forces controlling the abundance and distribution of fish across a landscape has been challenging, in part because few techniques exist to address multiple factors simultaneously in a single analysis. We used spatially explicit, varying-coefficient generalized additive models to relate the summertime abundance and distribution of yellow perch Perca flavescens to a variety of predatory, water quality, landscape, and density-dependent factors. The model used 25 years of fishery-independent trawling data from southern Green Bay, Lake Michigan, an area that supported major fisheries for yellow perch until their decline in the 1990s. Local catch per unit effort (CPUE) of both age-0 and age-1 and older yellow perch was affected by the abundance of double-crested cormorants Phalacrocorax auritus, dissolved oxygen, water clarity, and bottom depth, but not water temperature. In addition, the local response of age-0 yellow perch CPUE to most predictor variables, including their own g...