Gill and intestinal Na+-K+ ATPase activity, and estimated maximal osmoregulatory costs, in three high-energy-demand teleosts: yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwonus pelamis), and dolphin fish (Coryphaena hippurus)

We hypothesize that the morpho-physiological adaptations that permit tunas to achieve maximum metabolic rates (MMR) that are more than double those of other active fishes should result in high water and ion flux rates across the gills and concomitant high osmoregulatory costs. The high standard metabolic rates (SMR) of tunas and dolphin fish may, therefore, be due to the elevated rates of energy expenditure for osmoregulation (i.e. teleosts capable of achieving exceptionally high MMR necessarily have SMR). Previous investigators have suggested a link between activity patterns and osmoregulatory costs based on Na⁺-K⁺ ATPase activity in the gills of active epipelagic and sluggish deep-sea fishes. Based on these observations, we conclude that high-energy-demand fishes (i.e. tunas and dolphin fish) should have exceptionally elevated gill and intestinal Na⁺-K⁺ ATPase activity reflecting their elevated rates of salt and water transfer. To test this idea and estimate osmoregulatory costs, we measured Na⁺-K⁺ ATPase activity (V _max) in homogenates of frozen samples taken from the gills and intestines of skipjack and yellowfin tunas, and the gills of dolphin fish. As a check of our procedures, we made similar measurements using tissues from hybrid red tilapia (Oreochromis mossambicus ×O. niloticus). Contrary to our supposition, we found no difference in Na⁺-K⁺ ATPase activity per unit mass of gill or intestine in these four species. We estimate the cost of osmoregulation to be at most 9% and 13% of the SMR in skipjack tuna and yellowfin tuna, respectively. Our results, therefore, do not support either of our original suppositions, and the cause(s) underlying the high SMR of tunas and dolphin fish remain unexplained. Received: 7 September 2000 / Accepted: 4 December 2000     

Sustainable fishing gear: the case of modified circle hooks in a Costa Rican longline fishery

Our research aims to identify longline fishing gear modifications that can improve fishing selectivity and reduce incidental capture of non-target species. Catch rates and anatomical hook locations (AHL) were compared when using a 14/0 standard ??control?? circle hook with a 0° offset and an experimental ??appendage?? hook in a Costa Rican longline fishery. With the appendage, the maximum dimension of the appendage hook was increased by 10% and the minimum dimension of the hook by 19%. A total of 1,811 marine animals were captured during five fishing trips. By taxonomic groups, sea turtles represented the largest total catch (27%), followed by sharks (26%), rays (25%), mahimahi (Coryphaena hippurus) (12%), and tunas and billfish (10%). Non-target and discard species, such as rays and sea turtles, accounted for over half of the total catch. Catch per unit effort (CPUE; number of individuals per 1,000 hooks) was higher with control hooks compared to appendage hooks for all species?? categories except rays; appendage hooks caught 52% fewer sea turtles and 23% fewer tunas and billfish than standard hooks, which represents a significant reduction in bycatch of endangered and other species. No differences were found in the AHL for sea turtles, suggesting use of the appendage may not incur additional advantages regarding turtles?? post-release survivorship. Despite lower catch rates for marketable species, such as sharks and mahimahi, use of the appendage resulted in dramatic reductions in catch rates of sea turtles. The results suggest that large scale adoption of hooks with a significantly wider hook dimension could be an effective conservation measure to maintain marine biodiversity while allowing for continued fishing.