Ecological Risk Assessment and Productivity - Susceptibility Analysis of sea turtles overlapping with fisheries in the IOTC region
Interactions between sea turtles and fishing activities have been listed as a significant threat to sea turtles. This study aimed to assess which sea turtle species/populations in the Indian Ocean (IO) are at risk from interactions with tuna-related fisheries. The approach used was a desktop study to compile (1) all available data on sea turtle population demographics, rookery sizes and at-sea distributions; and (2) collate all information of longline, purse seine and gillnet effort and sea turtle interactions in the Indian Ocean.
A paucity of data on fishing effort for certain gear types, bycatch rates and sea turtle life history militated against a fully quantitative ecological risk assessment approach, and hence a semi-quantitative, categorical scoring approach was adopted to assess the relative risks of different gear types to different sea turtle species and populations. Combining all population demographic information with rookery size information, and rating each category as low (1), medium (2) and high (3) productivity, allowed for species-independent productivity scores (P) to be generated. The available fishing effort and spatial distribution of tuna-related fisheries, plus other species-specific attributes (such as turtle distributions) were used to rate the susceptibility of each sea turtle population to being caught per fisheries gear type (longlines, purse seine, and gillnets). The likelihood of being caught (as Susceptibility, S) was also rated according to low
(1), medium (2) and high (3). An overall Euclidian value rating vulnerability (V) to each of the three fisheries was obtained per sea turtle population in the IOTC region.
In total, 20 populations or regional management units (RMUs) were identified for the six species of sea turtles across the Indian Ocean. Satellite tracking information indicated that sea turtles occur at high densities in coastal (neritic) waters. However, these data are heavily biased towards tagged post-nesting female distributions. The distributions do however reflect the ‘high value’, breeding age-classes (i.e. sub-adults and adults).
Limited data on sea turtle bycatch (numbers and rates) were obtained from participating countries, with total data contributions constituting three longline data sets, one summary, and one report on purse seine activities. In the absence of fishing effort or turtle bycatch data in gillnets, catches (and bycatch) were inferred.
From the limited data on longlining and purse seining received, the former posed the greater apparent risk to sea turtles. We estimate that ~3,500 turtles.y-1 are caught in longlines, followed by ~250 turtles.y-1 in purse seine operations. For gillnetting, after the extensive literature survey, and recognising the important differences between artisanal and commercial gillnetting and between drift and anchored gillnets, we were forced to lump all gillnet data into a single category. Using the two approaches to estimate gillnet impacts on sea turtles, we calculated ~ 52,425 turtles.y-1 and 11,400–47,500 turtles.y-1 are caught in gillnets (with a mean of the two methods being
29,488 turtles.y-1). These values do not seem unrealistic as anecdotal/published studies reported values of >5000–16 000 turtles.y-1 for each of just India, Sri Lanka and Madagascar. Of these reports, green turtles are under the greatest pressure from gillnet fishing, constituting 50-88% of catches. Loggerhead, hawksbill and olive ridley turtles are caught in varying proportions depending on the region.
The Ecological Risk Assessment (ERA) methodology requires that where data are missing, a precautionary approach is adopted and a low productivity or high risk score assigned. The highest vulnerability ratings were obtained for data deficient species or small RMUs. Results were mixed with no particular gear type or species rating as consistently highly vulnerable. Generalising though, it seems like loggerheads have mixed vulnerabilities but the small RMUs (i.e. Bay of Bengal, BoB and
South Western Indian Ocean, SWIO) are vulnerable to all fisheries types but in particular gillnets. Green turtles are generally the least vulnerable as they have the largest populations, but are still vulnerable to gillnetting in the Arabian Gulf (AG). All three leatherback turtle RMUs (southwest
Indian Ocean, Bay of Bengal and South Pacific) are small and hence vulnerable to all fishing pressures. Similarly, small populations of hawksbill turtles (like the East Central Indian Ocean) are vulnerable to all fisheries (particularly gillnetting) whereas hawksbill turtles in the Arabian Gulf and the SWIO are reasonably balanced by rookery size and pressure. Olive ridley turtles have low productivity scores (mostly as a result of data deficiencies) but from the reports do not seem to interact with the reported fisheries. However, the data paucity is a great concern so there is low confidence in this result. The information available for flatback turtles in the South East Indian Ocean suggests that this population can sustain the current fishing pressures: the RMU is large, with an increasing trend and few reports of interactions with fisheries.
High priorities for future work include nesting beach demographic information, distribution of non-breeding size classes (juveniles and males), detailed demographic information from captured turtles (e.g. sex, size and species), as well as post-release survival rates. It was encouraging to note the large number of sea turtle action plans (and other exemplary practice) developing across the region.