Evaluation of the effectiveness of marine reserves for transient spawning aggregations in data-limited situations

Many coral reef fish species form predictable, transient spawning aggregations. Many aggregations are overfished, making them a target for spatial management. Here, we develop a per-recruit model to evaluate the performance of no-take marine reserves protecting transient spawning aggregations. The model consists of only 14 demographic and exploitation-related parameters. We applied the model to a protogynous grouper and a gonochoristic rabbitfish from Seychelles and tested six scenarios regarding the extent of protected areas, the level of fish spawning-site fidelity, and fishing effort redistribution post reserve implementation. Spawning aggregation reserves improve spawning-stock biomass-per-recruit and reduce the sex ratio bias in protogynous populations for all scenarios examined. However, these benefits are often small and vary among the different scenarios and as a function of sexual ontogeny. In all scenarios, increases in yield-per-recruit do not occur or are negligible. The long-term yield increases due to spawning aggregation reserves may still occur, but only if spawning-stock biomass recovery results in a recruitment subsidy. Given these limited benefits, the value of no-take reserves must be weighed against those of other management options, such as fishing effort reduction and seasonal fishery closures. The latter is particularly appropriate when spawning and non-spawning areas overlap in space

Shining a light on the composition and distribution patterns of mesophotic and subphotic fish communities in Hawai‘i

As agencies shift from single-species management to ecosystem-based fisheries management, ecosystem models are gaining interest for understanding species dynamics in relation to oceanographic and ecological processes and human marine uses. However, information on community structure or distribution of many species that occupy deep (>30 m) waters is largely unavailable. We amassed a total of 24 686 fish observations of 523 species/taxa for the 30−410 m depth areas surrounding the main Hawaiian Islands (MHI). We also obtained estimates of geomorphological variables, including substrate type, slope, rugosity, and ridge-like features. Using these 2 data sources, we (1) identified distinct fish communities along the 30−410 m depth gradient, and (2) generated relative biomass maps for fish functional groups. We showed that the mesophotic zone ranges between 30 and 129 m, with a fish faunal break at 60 m. Beyond this zone, 4 subphotic zones were identified: upper rariphotic (130−169 m), mid-rariphotic (170−239 m), lower rariphotic (240−319 m), and upper bathyal (320−410 m). We assigned fish species to functional groups partly based on identified depth ranges and fitted general additive models (GAMs) integrating geomorphological covariates to the functional group relative biomass estimates to determine the environmental variables that best predict the probability of encounter and relative biomass of each fish functional group. Finally, GAM predictions were employed to map functional group relative biomass distributions. These distribution maps showed a high relative biomass of many groups in the center of the MHI chain. This study contributes to a better understanding of fish community structure around the MHI and will inform ecosystem model parameterization

Warm and cold temperatures limit the maximum body length of teleost fishes across a latitudinal gradient in Norwegian waters

publishedVersio

A revised diet matrix to improve the parameterization of a West Florida Shelf Ecopath model for understanding harmful algal bloom impacts

Harmful algal blooms (HABs) are a growing concern in the West Florida Shelf (WFS) region. An Ecopath with Ecosim (EwE) model of the WFS explicitly simulating HABs was previously developed to illuminate the potential impacts of blooms of the dinoflagellate Karenia brevis (colloquially referred to as “red tides”) on the WFS ecosystem. However, the diet matrix of the Ecopath component of this EwE model (referred to as “WFS-HAB Ecopath”) was based largely on sparse, cursory information and not on local survey data. Here, we revise the diet matrix of the WFS-HAB Ecopath model using predictions of a robust statistical model that incorporates local survey data and employs the Dirichlet distribution and maximum likelihood estimation. The relative impacts of both the revised diet matrix and red tide mortality scenarios on model structure are explored by comparing four alternative WFS-HAB Ecopath models: (i) the base model; (ii) a model employing the revised diet matrix; (iii) a model with elevated red tide mortality; and (iv) a model with both the revised diet matrix and elevated red tide mortality. Incorporating the revised diet matrix into the WFS-HAB Ecopath model had a relatively large impact on ecosystem structure (i.e., trophic organization, mortality rates, trophic interaction strengths, and omnivory). Elevated red tide mortality had virtually no impact on ecosystem structure aside from altering the contribution of fishing, natural, and red tide mortalities to the total mortality of functional groups; however, elevated red tide mortality might have meaningful implications in dynamic simulations, which should be explored in future studies. Collectively, results showed that incorporating the revised diet matrix into WFS-HAB Ecopath, which revealed a number of new predator-prey linkages, led to a more complex and interconnected food web. Specifically, prey items were generally consumed by a broader variety of predators, which contrasts with the base WFS-HAB Ecopath model where many prey, particularly juvenile fishes, were subjected to exceedingly high predation mortality rates from specific predators. The incorporation of the revised diet matrix into the WFS-HAB Ecopath model discussed herein is a fundamental step towards increasing the realism of trophic interactions in the model, which is particularly important as these trophic interactions define starting conditions for dynamic simulations.publishedVersio

Soft-bottom fishes and spatial protection: findings from a temperate marine protected area

Numerous studies over the last decades have focused on marine protected areas (MPAs) and their effects on fish communities. However, there is a knowledge gap regarding how species that live associated with soft-substrates (e.g., sand, mud) respond to spatial protection. We analyzed abundance, biomass and total lengths of the soft-bottom fishes in a multiple-use MPA in the north-eastern Atlantic, the Luiz Saldanha Marine Park (Portugal), during and after the implementation of its management plan. Data were collected by experimental fishing in areas with three different levels of protection, during the implementation period and for three years after full implementation of the MPA. Univariate analysis detected significant biomass increases between the two periods. Fish assemblages were mainly structured by depth and substrate, followed by protection level. Community composition analyses revealed significant differences between protection levels and between the two periods. Species exhibited a broad variation in their response to protection, and we hypothesize that factors such as species habitat preferences, body size and late maturity might be underlying determinants. Overall, this study provides some evidence of protection effectiveness in soft-bottom fish communities, supported by the significant increase in biomass in the protected areas and the positive trends of some species.project LIFE-BIOMARES [LIFE06 NAT/P/000192]; project BUFFER (ERA-Net BiodivERsA); company SECIL-Companhia Geral de Cal e Cimento S.A.; FCT-Foundation for Science and Technology [CCMAR/Multi/04326/2013, SFRH/BD/80771/2011]; Foundation for Science and Technology [SFRH/BD/80771/2011]; 2012 Sesimbra Scientific Priz

The influence of sample distribution on growth model output for a highly-exploited marine fish, the Gulf Corvina (Cynoscion othonopterus)

Estimating the growth of fishes is critical to understanding their life history and conducting fisheries assessments. It is imperative to sufficiently sample each size and age class of fishes to construct models that accurately reflect biological growth patterns, but this may be a challenging endeavor for highly-exploited species in which older fish are rare. Here, we use the Gulf Corvina (Cynoscion othonopterus), a vulnerable marine fish that has been persistently overfished for two decades, as a model species to compare the performance of several growth models. We fit the von Bertalanffy, Gompertz, logistic, Schnute, and Schnute–Richards growth models to length-at-age data by nonlinear least squares regression and used simple indicators to reveal biased data and ensure our results were biologically feasible. We then explored the consequences of selecting a biased growth model with a per-recruit model that estimated female spawning-stock-biomass-per-recruit and yield-per-recruit. Based on statistics alone, we found that the Schnute–Richards model described our data best. However, it was evident that our data were biased by a bimodal distribution of samples and underrepresentation of large, old individuals, and we found the Schnute–Richards model output to be biologically implausible. By simulating an equal distribution of samples across all age classes, we found that sample distribution distinctly influenced model output for all growth models tested. Consequently, we determined that the growth pattern of the Gulf Corvina was best described by the von Bertalanffy growth model, which was the most robust to biased data, comparable across studies, and statistically comparable to the Schnute–Richards model. Growth model selection had important consequences for assessment, as the per-recruit model employing the Schnute–Richards model fit to raw data predicted the stock to be in a much healthier state than per-recruit models employing other growth models. Our results serve as a reminder of the importance of complete sampling of all size and age classes when possible and transparent identification of biased data when complete sampling is not possible

The influence of environmental factors and fishing effort on demersal fish species in Ghanaian waters

Disentangling the effects of fishing and the environment on fish abundance and distribution is essential for informing future fisheries management measures. Fishery-independent trawl survey data (8 years data between 1999 and 2016) were examined to test influences of fishing effort and environmental variables (temperature, oxygen salinity) on the spatial distribution of two species groups (5 economically important species and 3 non-commercial species) on the continental shelf of Ghana. Fishing effort influenced year-to-year variability in the abundance of 5 species negatively and 2 species positively, with 1 species unaffected. All species showed significant spatio-temporal associations with temperature, salinity and oxygen levels within the region. We observed some interannual consistency in fish spatial distributions given climatic variables’ correlation; however, some variability appeared to reflect tracking of year-to-year shifts in climatic variables, such as inshore–offshore shifts in goatfish, red pandora and red cornetfish associated with thermal and oxygen shifts. While the habitat models did not entirely explain the variability in these spatiotemporal patterns, overall, both commercially targeted and non-targeted demersal species appeared to be impacted by fishing and the species also track spatial and temporal changes in environmental conditions from year-to-year. Future fisheries management regulations in the region should incorporate spatially resolved measures of fishing effort alongside measures of environmental variables

Relative Impacts of Adult Movement, Larval Dispersal and Harvester Movement on the Effectiveness of Reserve Networks

Movement of individuals is a critical factor determining the effectiveness of reserve networks. Marine reserves have historically been used for the management of species that are sedentary as adults, and, therefore, larval dispersal has been a major focus of marine-reserve research. The push to use marine reserves for managing pelagic and demersal species poses significant questions regarding their utility for highly-mobile species. Here, a simple conceptual metapopulation model is developed to provide a rigorous comparison of the functioning of reserve networks for populations with different admixtures of larval dispersal and adult movement in a home range. We find that adult movement produces significantly lower persistence than larval dispersal, all other factors being equal. Furthermore, redistribution of harvest effort previously in reserves to remaining fished areas (‘fishery squeeze’) and fishing along reserve borders (‘fishing-the-line’) considerably reduce persistence and harvests for populations mobile as adults, while they only marginally changes results for populations with dispersing larvae. Our results also indicate that adult home-range movement and larval dispersal are not simply additive processes, but rather that populations possessing both modes of movement have lower persistence than equivalent populations having the same amount of ‘total movement’ (sum of larval and adult movement spatial scales) in either larval dispersal or adult movement alone

Relative mobility determines the efficacy of MPAs in a two species mixed fishery with conflicting management objectives

Marine Protected Areas (MPAs) have been used to protect species in need of conservation and as a fisheries management tool. It has been suggested MPAs can benefit mobile stocks by protecting spawning grounds whilst also allowing yields to be maintained as mature fish move out of the protected areas. However, the robustness of this claim in mixed species fisheries has yet to be established. We use a simulation model to explore the efficacy of spatial closures and effort regulation when other forms of fishery control (e.g., Total Allowable Catches) are absent or non-enforced as ways of addressing management objectives that are difficult to reconcile due to the contrasting life-histories of a target and a bycatch, conservation species in a two-species fishery. The mobility of each stock in such a fishery affects the benefits conferred by an MPA. The differing management objectives of the two species can be partially met by effort regulations or closures when the species exhibit similar mobility. However, a more mobile conservation species prevents both sets of aims being met by either management tool. We use simulations to explore how spatial closures and effort regulation can be used to seek compromise between stakeholders when the mobility of one stock prevents conflicting management objectives to be fully met. Our results demonstrate that stock mobility is a key factor in considering whether an MPA can meet conflicting aims in a multispecies fishery compromised of stocks with differing life histories and mobilities