Modeling the population dynamics of lemon sharks

Transactions of the American Fisheries Society, 2001

The survival rate of juvenile lemon sharks Negaprion brevirostris in North Sound, Bimini, Bahamas, was estimated by marking a cohort of small sharks (Յ53 cm precaudal length) each spring from 1995 to 1999 and by estimating the number of survivors 1 year later by using a depletion method on the marked population. Annual survival rate estimates varied between 38% and 65%, which was somewhat higher than the 39% steady-state survival predicted from a life cycle (Leslie matrix) model. These are the first direct estimates of the survival rate of a juvenile elasmobranch, and the results support modeling efforts that have been used in determining limits to sustainable exploitation of elasmobranchs. Estimates of survival rates in the present study were negatively correlated with estimated initial abundance and are consistent with strong, densitydependent survival.

2002

The status of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) was assessed using a Bayesian agestructured population model. The results of the model indicated that the best estimate of the mature female biomass in 2000 was 69.6% of the 1972 biomass. Sensitivity tests indicated that changes in data on catches, catch rates and biology did not significantly change the estimates of biomass. The results of risk assessment indicated that the probability of depletion of the population to below 5% or 30% of the 1972 level by 2020 is low, even with substantial increases in future catches. The results show a high level of uncertainty, indicating a need to improve the quality of data collected if assessments are to be improved. The implications of the assessment results for fisheries management are discussed.

Environmental Biology of Fishes, 2008

The waters around Bimini (25°43.70′ N, 79°1 8.00′ W) provide an ideal nursery location for juvenile lemon sharks, Negaprion brevirostris, but this habitat is threatened by the development of a large resort. Since 1999 the North Sound (NS) has been subjected to intermittent periods of dredging, the most intensive of which was in March 2001. Possible effects from the development up to June 2006 were investigated by: comparing growth rates of juvenile lemon sharks in the NS, Sharkland (SL) and South Bimini (SB) nurseries between 1995-2005 using before-after, control-impact (BACI) analysis; analyzing survival of juvenile lemon sharks in the NS and SL between 1995-2006; and by comparing habitat structures in the NS and SB nurseries in 2003 and 2005. BACI analysis detected no statistically significant difference between the growth rates of juvenile lemon sharks in the three nurseries before and after the impact date of March 2001. However, a reduction in the survival rate of juvenile lemon sharks in the NS after March 2001 was statistically significant, including a 23.5% decline in first-year survival. Habitat structure of the NS in 2003 and 2005 also varied with the mean percentage cover of the seagrass Thalassia testudinum declining by 17.7% since 2003.

The blue shark, Prionace glauca, is a pelagic species with a circum-global distribution in tropical to temperate waters. It is the top by-catch shark species for the tuna longline fishery in the open seas. However, its population status in the North Pacific Ocean is still unknown. The intrinsic rate of population increase (r) is an important and crucial parameter in fish stock assessment. Hence, the r of the blue shark in the North Pacific Ocean was estimated using a demography analysis in this study. The input life history parameters including the growth coefficient, longevity, fecundity, age at maturity, reproductive cycle, and natural mortality were adopted from the best information available. The results of demographic analysis indicated that the average r of blue shark ranges from 0.138 to 0.513 yr-1 in natural condition suggesting that this species is very productive. Furthermore, the results obtained from different cases demonstrated that the natural mortality and reproductive cycle have the highest sensitivity. Future work should focus on accurate estimate of natural mortality and reproductive cycle for this species to improve the demographic estimates. The results derived from this study can be used as the prior of Bayesian surplus production model of blue shark in the North Pacific Ocean. This approach can also be applied to other species on the estimation of demographic parameters.

PLOS One, 2011

Overfishing of sharks is a global concern, with increasing numbers of species threatened by overfishing. For many sharks, both catch rates and underwater visual surveys have been criticized as indices of abundance. In this context, estimation of population trends using individual demographic rates provides an important alternative means of assessing population status. However, such estimates involve uncertainties that must be appropriately characterized to credibly and effectively inform conservation efforts and management. Incorporating uncertainties into population assessment is especially important when key demographic rates are obtained via indirect methods, as is often the case for mortality rates of marine organisms subject to fishing. Here, focusing on two reef shark species on the Great Barrier Reef, Australia, we estimated natural and total mortality rates using several indirect methods, and determined the population growth rates resulting from each. We used bootstrapping to quantify the uncertainty associated with each estimate, and to evaluate the extent of agreement between estimates. Multiple models produced highly concordant natural and total mortality rates, and associated population growth rates, once the uncertainties associated with the individual estimates were taken into account. Consensus estimates of natural and total population growth across multiple models support the hypothesis that these species are declining rapidly due to fishing, in contrast to conclusions previously drawn from catch rate trends. Moreover, quantitative projections of abundance differences on fished versus unfished reefs, based on the population growth rate estimates, are comparable to those found in previous studies using underwater visual surveys. These findings appear to justify management actions to substantially reduce the fishing mortality of reef sharks. They also highlight the potential utility of rigorously characterizing uncertainty, and applying multiple assessment methods, to obtain robust estimates of population trends in species threatened by overfishing.

Molecular …, 2009

Although many sharks begin their life confined in nursery habitats, it is unknown how rapidly they disperse away from their natal area once they leave the nursery. We examine this issue in immature lemon sharks (Negaprion brevirostris) from the time they leave the nursery ($ age 3) at a subtropical island (Bimini, Bahamas), through to the onset of sexual maturity ($ age 12). From 1995 to 2007 we tagged and genotyped a large fraction of the nursery-bound sharks at this location (0-3 years of age, N = 1776 individuals). From 2003 to 2007 we sampled immature sharks aged from 3 to 11 years (N = 150) living around the island and used physical ⁄ genetic tag recaptures coupled with kinship analysis to determine whether or not each of these 'large immature sharks' was locally born. We show that many island-born lemon sharks remain close to their natal area for long periods (years) after leaving the nursery; more than half of the sampled sharks up to 135 cm total length ($6 years old) were locally born. The fraction of locally born sharks gradually declined with increasing shark size, indicating that dispersal is relatively slow and does not primarily occur after sharks reach a threshold size. Local conservation measures (e.g. localized fishery closures, marine protected areas) can therefore help protect island-born lemon sharks even after they leave the nursery habitat.

ICES Journal of Marine Science, 2014

Most demographic models are single sex, and assume both sexes have the same vital rates. However, many species, including the shortfin mako shark, are sexually dimorphic in vital rates, which suggests the need for two-sex models. In this study, a two-sex stage-structured matrix model was constructed to estimate shortfin mako shark demography and population dynamics. Monte Carlo simulations were used to evaluate the impact of uncertainties on the estimate of population growth rate. The number of shortfin mako sharks is found to be dropping under current conditions, but will stabilize if size-limit management is implemented. The simulations indicated that population growth rate estimates are mainly influenced by the uncertainty related to survival rate and fecundity. The effects of uncertainty regarding the age at maturity and longevity were found to be relatively minor. Future research should focus on obtaining estimates of natural mortality and reproductive traits for this species t...

Fisheries Research, 2010

We estimate recent (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) trends in relative abundance for Northwest Atlantic oceanic and large coastal sharks, using generalized linear mixed models to standardize catch rates of eight species groups as recorded by U.S. pelagic longline fishery observers. Models suggest precipitous (76%) declines in hammerhead (Sphyrna species) and large coastal (dusky, night, and silky shark, genus Carcharhinus) species, and moderate declines (53%) in blue and oceanic whitetip sharks over this period. In contrast, mako and thresher sharks appear to have stabilized, and the tiger shark population appears to be increasing. A comparison of nominal shark catch rates from this fleet's observer and logbook data (to evaluate the veracity of trends previously estimated from the latter) showed a high degree of concordance for each species group, both in individual sub-areas and overall. Models of these two datasets for the common time period (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) show that compared to the observer data the logbook data indicate greater declines for some species, but lesser declines for others. Signs of recovery for some shark species are encouraging, but must also be set in the context of the significant declines that occurred in previous decades.

Environmental Biology of Fishes, 2005

We compared growth rates of the lemon shark, Negaprion brevirostris, from Bimini, Bahamas and the Marquesas Keys (MK), Florida using data obtained in a multi-year annual census. We marked new neonate and juvenile sharks with unique electronic identity tags in Bimini and in the MK we tagged neonate and juvenile sharks. Sharks were tagged with tiny, subcutaneous transponders, a type of tagging thought to cause little, if any disruption to normal growth patterns when compared to conventional external tagging. Within the first 2 years of this project, no age data were recorded for sharks caught for the first time in Bimini. Therefore, we applied and tested two methods of age analysis: (1) a modified 'minimum convex polygon' method and (2) a new age-assigning method, the 'cut-off technique'. The cut-off technique proved to be the more suitable one, enabling us to identify the age of 134 of the 642 previously unknown aged sharks. This maximised the usable growth data included in our analysis. Annual absolute growth rates of juvenile, nursery-bound lemon sharks were almost constant for the two Bimini nurseries and can be best described by a simple linear model (growth data was only available for age-0 sharks in the MK). Annual absolute growth for age-0 sharks was much greater in the MK than in either the North Sound (NS) and Shark Land (SL) at Bimini. Growth of SL sharks was significantly faster during the first 2 years of life than of the sharks in the NS population. However, in MK, only growth in the first year was considered to be reliably estimated due to low recapture rates. Analyses indicated no significant differences in growth rates between males and females for any area. Environmental Biology of Fishes (2005) 72: 343-355 Ó Springer 2005

Col. Vol. Sci. Pap. …, 2005

The paucity of exploitation and biological information for many pelagic shark species, such as blue shark (Prionace glauca), often thwarts the use of detailed population dynamics models for the simulation of the dynamics of those species and assessment of their status. A detailed ...