Potential Impacts Of Fish Aggregation Devices(FADs) On Feeding Patterns For Skipjack Katsuwonus Pelamis In The Western And Central Pacific Ocean

The tropical tunas have the habit of gathering around natural floating objects such as logs, seaweed mats, and palm leaves, forming stable aggregations, termed as floating-objects-associated schools, which easy to be caught. Tuna purse seine fisheries utilized this type of behavior to develop a great number of fish aggregation devices(FAD) for aggregating and catch tunas. Skipjack tuna Katsuwonus pelamis is the most important target species in fisheries of tuna purse seine in Western and Central Pacific Ocean. FAD have been generally accepted as an effective mean to increase catches and catch rates of purse Skipjack. The large-scale use of FAD, however, considerably increases the density of floating objects in some waters, and in turn, to some extent, artificially changes the surface habitat of the tuna. This may impose a series of potentially negative effects on Skipjack populations, such as changes in patterns of aggregation, feeding pattern and migratory movement and the decline of physical conditions for the population. Of those mentioned-above side effects, changes in the feeding pattern may have more profound impacts on fish populations and should be consideredBased on the samples of Skipjack from free swimming schools and FAD-associated schools collected by tuna purse seine “Jin Hui number 7” operating in the Western and Central Pacific Ocean during February 2013 to March 2013 and February 2014 to April 2014. This study was to analyze stomach contents and stable isotope component of muscle tissue by which the similarities and differences in the diet of species, sizes, carbon and nitrogen stable isotope of Skipjack from these two schools can be compared, consequently assess the potential impacts of Fish Aggregation Devices(FAD) on feeding patterns for skipjack in the Western and Central Pacific Ocean.The main results can be found as follow,(1) With the increase of the fork length, the frequency of Skipjack increases first and then decrease both in free swimming schools and FAD-associated schools. 50.0-59.9cm is the advantage fork length class for free swimming schools, but 40.0-49.9cm for FAD-associated schools, taking up 49.2% and 37% of total amount respectively.(2) The proportion of empty stomach is 16.9% for free swimming schools, while 97.2% for FAD-associated schools. The mean daily ration of Skipjack from free swimming schools decreases with fork length class. The maximum average daily ration is 1.13%±0.75 appears on Skipjack of fork length at 40.0-49.9cm from free swimming schools.(3) From stomach contents of free swimming schools, it was found that the main bait is Stolephorus buccaneeri which makes up 97.07 % in weight and 99.77 % in quantity. In 86.11 % Skipjack, the stomach contents contain that species exclusively. Skipjack from free swimming schools preference to feeding Stolephorus buccaneeri of 2.1-5.0cm.(4) According to the ANOVA test, the fork length has a significant effect on δ13C value both in free swimming schools and FAD-associated schools(P<0.05).The δ13C value declines with the increase of the fork length class in both schools. It indicates that there have some differences in primary diet source for Skipjack in different fork length.The carbon stable isotope between two schools has a significant difference through the range of fork length(P<0.05). The mean δ13C value of FAD-associated schools appears slightly higher than that of free swimming schools. But statistically, the δ13C value was no significant difference in other fork length class in two schools(P>0.05), except in fork length class of 50.0-59.9 cm.(5) It is shown by the ANOVA test, the fork length imposes in significant effect on δ15N value in free swimming schools(P>0.05), but it exerts a significant effect on δ15N value in FAD-associated schools(P<0.05). The δ15N value of both free swimming schools and FAD-associated schools declines with the increase of the fork length. The mean δ15N value of FAD-associated schools appears higher than that of free swimming schools in 30.0-39.9cm fork length class, but with the opposite results in the rest of fork length class.(6) There is no obvious difference of the mean trophic position value among each fork length class in free swimming schools. While in FAD-associated schools, the mean trophic position value decreases first and then keeps constant with increase of the fork length, and the range of maximum and minimum is less than 1. By comparing the mean trophic position value of free swimming schools and FAD-associated schools in each fork length class, the results show that in each fork length class, the mean trophic position in two schools appears no significant difference(P>0.05).(7) No correlation is observed between δ15N and δ13C in free swimming schools(P> 0.05), but there is a significant positive correlation(P <0.05) between δ15N and δ13C in FAD-associated schools. The space of 95% confidence interval of δ15N andδ13C position in FAD-associated schools is significant larger than that in the free swimming schools. It means that the niche width in FAD-associated schools is greater than that in the free swimming schools.(8) ANOVA test showed that the value of the fork length exerts insignificant influence on the C: N ratio(P> 0.05) in free swimming schools, while exerts significant influence in FAD-associated schools(P <0.05). The mean C:N value of each fork length class has no obvious difference in both schools. The mean C:N value at the same fork length class, however, all appear less in free swimming schools than in FAD-associated schools...