Robert Ahrens

Erratum: Simulating spatial dynamics to evaluate methods of deriving abundance indices for tropical tunas

Relative abundance indices derived from nominal catch-per-unit-effort (CPUE) data are a principle source of information for the majority of stock assessments. A particular problem with formulating such abundance indices for pelagic species such as tuna is the interpretation of CPUE data from fleets that have changed distribution over time. In this research, spatial population dynamics are simulated to test the historical pattern of fishing effort as a basis for making inferences about relative abundance. A number of age-structured, spatially disaggregated population dynamics models are described for both Atlantic yellowfin tuna (Thunnus albacares) and bigeye tuna (Thunnus obesus) to account for uncertainty in spatial distribution and movement. These models are used to evaluate the reliability of standardization methods and a commonly applied model selection criterion, Akaike’s information criterion (AIC). The simulations demonstrate the pitfalls of aggregating CPUE data over spatial areas and highlight th...

Hide and Seek: Interplay of Fish and Anglers Influences Spatial Fisheries Management

ABSTRACTSustainable management of fisheries resources requires an understanding of spatial and temporal interplay between targeted fish populations and anglers. We conducted a field study comparing spatial patterns in recreational angler effort to fish distribution in a Florida lake. Over one year, spatial locations of Largemouth Bass (Micropterus salmoides) anglers and Largemouth Bass were surveyed. Over 90% of anglers were fishing within 50 m from shore and one-third of fish were located offshore at any given time. This spatial patterning suggested that fish located in areas not targeted by anglers were less vulnerable to angling and, thus, anglers were not distributed according to the ideal free distribution. However, tag return data of telemetered fish showed similar catch trends in both onshore and offshore habitats, indicating that all fish were equally vulnerable to angling and anglers were ideally distributed. Informed use of spatial and/or temporal fishery regulations should consider fish and ang...

Potentials and Limitations of Stock Enhancement in Marine Recreational Fisheries Systems: An Integrative Review of Florida's Red Drum Enhancement

In this study, an integrative review of the potential for stock enhancement is conducted to support desirable management outcomes in marine recreational fisheries, focusing on the Florida, USA, red drum fishery as a case study. Here, stock enhancement is implicitly seen as a way of simultaneously achieving both ecological objectives of sustained wild fish populations and socioeconomic objectives of high fishing effort and/or catch rates. However, the review suggests that a fundamental tradeoff remains between these objectives in the short-term because stocking of hatchery fish is likely to result in at least partial displacement of wild fish through biological interactions as well as increased fishing pressure. Contrary to the perception of enhancement as a “quick fix,” successful use of the approach in the marine recreational fishery is likely to require sophisticated stock management and some adaptation in governance. In developing the enhancement, it will be necessary to address uncertainty in key attributes, specifically dynamics of recruitment, angler-effort responses, and stakeholder involvement. This may be achieved by combining quantitative modeling, monitoring, and stocking experiments in an active adaptive management framework to consider enhancement in the context of alternative management strategies. It is suggested that any interim enhancement should minimize ecological risk per socioeconomic benefit by stocking larger fish in areas where high fishing mortality limits abundance of wild fish. These conclusions are largely generalizable to other recreational enhancements, and this work serves as a model of rarely published a priori enhancement evaluation.

The curious case of eastern oyster Crassostrea virginica stock status in Apalachicola Bay, Florida

The Apalachicola Bay, Florida, eastern oyster (Crassostrea virginica) industry has annually produced about 10% of the U.S. oyster harvest. Today’s simple individual-operator, hand-tonging, small-vessel fishery is remarkably similar to the one that began in the 1800s. Unprecedented attention is currently being given to the status of oyster resources in Apalachicola Bay because this fishery has become central to the decision making related to multistate water disputes in the southeastern United States, as well as millions of dollars in funding for restoration programs related to the Deepwater Horizon oil spill. The oyster fishery collapsed in 2012, leading to large economic losses and community concerns over the current and future status of oyster resources, ecosystem health, and local economic opportunities. We used best available data to assess what mechanism(s) may have led to the collapse of the Apalachicola Bay oyster fishery. We then assessed the efficacy of alternative management strategies (e.g., restoration, fishery closure) to accelerate oyster population recovery. Our results suggest that the Apalachicola Bay oyster population is not overfished in the sense that recruitment has been limited by harvest, but that the 2012 collapse was driven by lower-than-average numbers and/or poor survival of juvenile oysters in the years preceding the collapse. This reduction in recruitment not only reduced the biomass of oysters available to harvest, but from a population resilience perspective, likely reduced the amount of dead shell material available as larval settlement area. Although the Apalachicola Bay oyster fishery has proven resilient over its >150-year history to periods of instability, this fishery now seems to be at a crossroads in terms of continued existence and possibly risks an irreversible collapse. How to use the restoration funds available, and which restoration and management practices to follow, are choices that will determine the long-term viability of the Apalachicola Bay oyster fishery.

Combining Samples from Multiple Gears Helps to Avoid Fishy Growth Curves

AbstractSize-at-age information is critical in estimating growth parameters (e.g., the von Bertalanffy growth function [VBGF]) that are used to assess fish populations. Due to gear selectivity, single sampling methods rarely sample all ages or all sizes equally well. Most growth estimates rely on samples from a single gear or a haphazard combination of gears, potentially leading to biased and imprecise growth parameter estimates. We evaluated the efficacy of combining samples from two gears with different size selectivity to estimate VBGF parameters; we then applied that approach to a case study on the Lochloosa Lake (Florida) population of Black Crappies Pomoxis nigromaculatus. Simulated age- and size-structured populations were randomly sampled with two gears characterized by different size-selectivity curves (one gear was selective for smaller fish; the other was selective for larger fish). Maximum likelihood VBGF estimates obtained for each gear separately were compared with estimates from a combined ...

Assessing the Utility of a Smartphone App for Recreational Fishery Catch Data

There is a need to improve the management of recreational fisheries, and some organizations have begun piloting and implementing opt-in, self-reporting smartphone and tablet apps to further that goal. We began the process of developing a methodology for assessing the utility of these apps for management by comparing mean catch/trip values of selected species between the Snook and Gamefish Foundations iAngler app and the Marine Recreational Information Program survey. The iAngler data set is used almost exclusively in Florida and is characterized by high spatial variability. However, its catch rates for Common Snook Centropomus undecimalis (medians ranging from 0 to 2), Spotted Seatrout Cynoscion nebulosus (medians from 1 to 3), and Red Drum Sciaenops ocellatus (medians from 0 to 1) were similar to those provided by the Marine Recreational Information Program (medians ranging from 1 to 2, 1 to 4, and 0 to 3, respectively). Self-reporting programs often suffer from biases concerning angler avidity, drop-of...

Use of underwater video to assess freshwater fish populations in dense submersed aquatic vegetation

Underwater video cameras (UVC) provide a non-lethal technique to sample fish in dense submersed aquatic vegetation. Fish often inhabit densely vegetated areas, but deficiencies of most sampling gears bias relative abundance estimates that inform fisheries management. This study developed methods using UVC to estimate relative abundance in dense vegetation using three experimental ponds covered with surface-matted hydrilla (Hydrilla verticillata) stocked at different densities of Lepomis spp. and largemouth bass (Micropterus salmoides). We conducted UVC point counts over 13 weeks to measure relative fish abundance and occurrence from video analysis. Ponds were then drained to obtain true fish densities. In total, fish were detected in 55% of all counts and juvenile and adult Lepomis spp. and largemouth bass were enumerated. End-of-season true fish densities ranged across ponds (from 52 to 37000 fishha � 1 ). Additionally, pond 2s true density changed substantially from 370 to 12300 fishha � 1 . True population size was accurately reflected in differences in estimated relative abundances obtained from fish counts, as in pond 2 where mean fish counts increased from 0.10 in week 1 to 2.33 by week 13. Underwater video accurately and precisely quantified relative abundance at naturally-occurring fish densities, but this success was reduced at low densities. Additional keywords: complex habitats, fish sampling, Hydrilla verticillata, invasive aquatic plants, largemouth bass, sunfish, video cameras.

Evaluating the efficacy of the Florida Keys’ angler-assisted permit tagging program

The use of angler-based tagging programs in recreational fisheries is on the rise, as they provide managers with effective, inexpensive means of collecting information. However, such programs are rarely evaluated for efficacy during the course of the program. Our study tagging program was the recreational permit Trachinotus falcatus (Linnaeus) fishery in the Florida Keys, which began in 2010. We evaluated the program for potential improvements using two complimentary approaches: an email-based survey of anglers and an advection–diffusion simulation model. Survey respondents reported potential fish stress and poor access to tags as the biggest obstacles to tagging permit. Respondents also cited poor data feedback from scientists as a potential area for improvement. According to the advection–diffusion simulation, the current level of tag returns is insufficient for accurately and precisely estimating movement parameters. The Florida permit tagging program could be greatly improved with greater communication between scientists and anglers. Data feedback and angler rewards could each increase angler buy-in. Tag retention could have a strong effect on the tag return rate and deserves more attention. Tag reporting could also have a strong impact on the tag return rate. The Florida permit tagging program needs more work to become a success, but has the potential to provide benefits for scientists and anglers.

Bonefish in South Florida: status, threats and research needs

Bonefish (Albula vulpes) support a world-renowned fishery in South Florida, USA. However, fishing guides and anglers have been reporting significant declines in bonefish angling quality over that past three decades. In the absence of any long-term bonefish stock and ecosystem assessments, the cause of this decline in the fishery is unclear. Here we summarize our current knowledge of bonefish ecology in Florida and discuss potential causes of fishery decline. Reductions and alterations in freshwater flows from the Everglades have caused major changes in bonefish habitat, including acute (anoxic conditions) and chronic (changes in benthic flora and fauna) effects in Florida Bay and Biscayne Bay. Various pollutants from agricultural and urban runoff may also be impacting bonefish population(s) directly and/or indirectly throughout their range. Efforts to locate juvenile A. vulpes in Florida have been largely unsuccessful to date, suggesting abundances may be low, and/or juveniles have unknown habitat requirements in Florida. Further, bonefish larvae may be sourced from adult individuals outside of Florida in areas such as Cuba or Mexico, in which case bonefish conservation in other regions is highly relevant to the Florida population. Extreme weather events may have also contributed to the decline; an extreme cold spell in 2010 caused significant bonefish mortality and coincided with documented declines in the fishery. The fishery may also be impacting the population. We outline research needs and potential approaches to better understand the causes of the bonefish decline in Florida and restore populations of this ecologically and socioeconomically important species.

Use of a dynamic population model to estimate mortality and recruitment trends for Bonefish in Florida bay

Bonefish (Albula vulpes) are popular fish for sports anglers in South Florida. Despite the implementation of catch and release policies, there is concern that numbers are declining. Previous work found a significant downward trend for Bonefish CPUE from 1980 to 2014, and a structural breakpoint in the year 2000. Our objective was to develop and use an age-structured model to explore the relative role of recruitment and changes to adult mortality to explain the decline. Parameters for Bonefish growth, mortality, fecundity and recruitment were compiled from previous research and literature. The CPUE time series from 1980 to 2014 was used to fit the model. Recruit survivorship was responsible for explaining most of the variation in the CPUE time series, while adult survivorship had a significant downward trend. There was no statistically significant structural breakpoint in the survivorship time series, but there was a shift in the mean values in 1998 for adult survivorship and 2008 for recruit survivorship. The shift in adult survivorship is close to the CPUE breakpoint and thus interpreted as driving that change. The fact that there is also a shift in recruit survivorship is cause for concern and warrants continued monitoring of Bonefish populations and research into spawning and recruitment. There is a need to identify biophysical, climatological and habitat variables that are responsible for recruitment and mortality anomalies, and the time series estimates generated in this study will be useful for the analysis of management and conservation decisions in relation to these variables.