Pacific mobulids: unwanted catch

The aim of this work is to describe and understand the distribution of manta and mobula species caught in the tropical purse seine fishery of the eastern Pacific Ocean and explore using this information to reduce by-catch.

Declines in manta and mobula populations appear to have been large in several regions, and a global decrease is strongly suspected in some species. The impacts of fisheries (through direct targeting and incidental mortality) are considered by many to be the main cause of these declines. In the eastern Pacific Ocean, an increase in catches of both manta and mobula rays has been identified as a problem because of the species’ vulnerability to over-exploitation. However, there are few studies about the effects of this type of fishing gear on their populations. An increase in fishing effort in recent years as a consequence of the growth in fleet capacity has resulted in an increase in tropical tuna catches and the catch of non-target species associated with them.

Manta and mobula rays are potentially migratory; they are all filter feeders, but in general, mobula rays are much smaller than manta rays and can be distinguished by their morphological features. Circumglobally distributed in tropical and temperate waters, mobula rays have a widespread distribution and they appear to be seasonal visitors to coastal and off-shore sites. However, their distribution is not well defined due to a lack of data. In general, they are often seen aggregating in large numbers in spring and summer to feed, mate or visit cleaning stations around the Gulf of California and west coast of Mexico. In the eastern Pacific, these rays are incidentally caught by fisheries and routinely returned to the sea.
Mobula rays live a long time and reproduce infrequently, so they are vulnerable to exploitation. Furthermore, the lack of extensive scientific information on the biology and ecology of these species is currently severely impeding their management and conservation. Observer programmes provide good information to inform conservation issues.

The aims and objectives of this project are to:

• Study the magnitude of manta and mobula ray catches in a spatial and temporal framework to evaluate the overall impact of the eastern Pacific Ocean tropical purse seine fishery. Differences in the sizes of the species and the type of fishing will be analysed to consider their relative significance for population dynamics.
• Determine the species and size diversity of the catches in different areas and during different time periods.
• Gain an understanding of the habitats used by different species in the oceanic environment using Maximum Entropy (MaxEnt) Modeling.
• Study changes in the distribution of these species as consequence of climate change.