Studies have shown that marine protected areas (MPAs) can be used as tools to preserve marine biodiversity, but currently there is little empirical evidence that MPAs have been effective for improving demographic parameters in marine mammal populations. In 1988, the Banks Peninsula Marine Mammal Sanctuary was established in New Zealand to decrease the number of gillnet mortalities of Hector’s dolphin, an endangered dolphin species. Gormley et al. (2012) hypothesized that reduced gillnetting pressures in this sanctuary should result in increased adult survival rate and an improvement in population growth. They addressed this hypothesis by applying random effects Bayesian modeling to photographic capture-recapture data collected through photo-identification studies that lasted for 21 year. They then specified a matrix protection model for pre- and post-sanctuary periods to carry out population projections. Gormley et al. estimate that there is a 90% probability that dolphin survival has improved since the creation of the sanctuary. They found that mean dolphin survival probability increased 5.4% which corresponds to a 6% increase in mean annual population growth, thus providing evidence that area-based protection measures can be effective for marine mammals. —Evelyn Byer
Gormley, A. M., Slooten, E., Dawson, S., Barker, R. J., Rayment, W., du Fresne, S. and Bräger, S. (2012), First evidence that marine protected areas can work for marine mammals. Journal of Applied Ecology, 49: 474–480. doi: 10.1111/j.1365–2664.2012.02121.x
Gormley and colleagues from New Zealand assessed dolphin survival probability by coupling models with field transect data. Photo-identification of Hector’s dolphins were conducted through standardized along-shore transects around Banks Peninsula in 4–6 m boats. All distinctive dolphins were photographed in each pod sighting before continuing the transect. Photographs of individual dolphins were only used if the dorsal fin was completely visible, in focus, and perpendicular to the photographer to ensure that identifying marks would be completely visible. A catalogue of identifiable individuals was maintained along with a data base containing individual sighting history. No photo-identification fieldwork was collected in 1998 or 1999. Data were restricted to captures during November to February (inclusive) to satisfy the assumption of population closure within each sampling period for the capture-recapture model. Gormley et al. used a modified form of the Cormack-Jolley-Seber (CJS) model to estimate annual survival rate. This model allows for imperfect detection and was implemented using a state-space modeling approach that includes a process model and an observation model. The CJS model was fitted in a Bayesian framework using WinBUGS. Three Markov-chains were started from different initial values and run for 10,000 iterations to tune the algorithm. These samples were then discarded and the algorithm was run for a further 100,000 samples. Convergence was assessed visually and goodness-of-fit was assessed using a posterior predictive checking procedure for a general CJS model. This capture-recapture analysis differs from the classical approach in that a single model is specified in which all the parameters are acknowledged to vary in a random manner with time about some mean. A fixed-duration, stage-structured matrix model was specified to carry out population projections and used three life stages: calf, junvenile, and adult.
During 1986–2006, a total of 462 reliably marked individuals were photographically captured during the summer periods. The results of Gormley et al.show a 90% probability that survival rate has improved between the pre- and post-sanctuary periods. This translates to a mean increase in annual survival rate of 5.4% since the establishment of the sanctuary. All but one (1991) of the point estimates of annual survival from the post-sanctuary period are greater than those from the pre-sanctuary period. These results suggest that the sanctuary’s restrictions on gillnetting have reduced the bycatch of Hector’s dolphins. Furthermore, the increase in survival corresponds to an increase in population growth of 6%. Gormley et al. studied survival rate and projected population growth rather than direct estimates of abundance because survival is typically estimated with less bias and greater precision. Although the post-sanctuary survival rate estimate of 0.917 is substantially higher than the pre-sanctuary rate (a 0.054 increase), this rate is too low to allow population recovery. Gormley and colleagues conclude that the Banks Peninsula Marine Mammal Sanctuary is too small to afford effective protection to the Hector’s dolphin population.