Scientific Journal Article Summary
Comparative influence of ocean conditions on yellowfin and Atlantic bluefin tuna catch from longlines in the Gulf of Mexico.
Steven L. H. Teo and Barbara A. Block
PLoS ONE (2010) 5: e10756. doi:10.1371/journal.pone.0010756
The Gulf of Mexico is the primary spawning area of the western Atlantic bluefin tuna. Recognizing the importance of protecting spawning fish in the ongoing effort to rebuild the population, targeted bluefin fishing has been banned in the Gulf for over twenty years. However, bluefin continue to be captured accidentally on pelagic longlines targeting yellowfin tuna and swordfish, often resulting in mortality. This study compares the variability of catch rates of bluefin and yellowfin tuna in time and space and explores the environmental influences on the catch rates in order to distinguish potential differences in distribution of bluefin and yellowfin catch in the Gulf of Mexico.
Using electronic tagging and fisheries catch data coupled with oceanographic datasets, a negative binomial model was developed to determine the relative probability of catching bluefin and yellowfin tuna at a given place and time. The model revealed pronounced differences in preferred habitat of Atlantic bluefin tuna and yellowfin tuna in the Gulf of Mexico, despite their close ancestry. Bluefin tuna return to the same regions of the Gulf of Mexico during spring months to spawn. While bluefin are captured in the Gulf from January through June each year, the highest catch rates are in April (0.472 bluefin per 1000 hooks) and May (0.427 bluefin per 1000 hooks) during the bluefin spawning season. There are two major hotspot regions within the Gulf where bluefin bycatch occurs – one in the eastern Gulf of Mexico to the north of the Loop Current, and the other in the western Gulf of Mexico. Both regions are along the slope where the shallow continental shelf changes rapidly to the deep sea. It is within these hotspots that bluefin tuna prefer to spawn in circular, swirling water masses called “cyclonic eddies.” These eddies are more productive and slightly cooler than surrounding warm Gulf ocean currents, and are predictable and can be observed from satellites. Yellowfin tuna are more widely distributed throughout the warm Gulf waters and occupy the region throughout the year; wind speed was the only environmental variable found to have a significant effect on yellowfin catch rates.
The differences in spatiotemporal distribution of bluefin and yellowfin tuna can be seen clearly in the maps below. Figure 1 shows the probability of encountering a bluefin tuna when fishing with a pelagic longline in the Gulf of Mexico, while Figure 2 shows the probability of encountering a yellowfin tuna. The probability of encounter is indicated by color from high probability (red) to low (blue). Figure 3 shows the mean catch per unit effort of bluefin (closed circles) and yellowfin (open circles) throughout the year.
Figure 1
Figure 2
Figure 3
Importantly, bluefin and yellowfin tuna catch rates were not siginficantly correlated between March and June. These findings indicate that it would be possible to utilize spatial management techniques to protect western Atlantic bluefin tuna on their breeding grounds in the Gulf of Mexico without compromising the yellowfin tuna fishery, which could be carried out in other areas during the critical bluefin tuna breeding times.