Characteristics Of The Habitat Depth And Fluctuation Of The Resources For Swordfish In The Atlantic Ocean

Swordfish (Xiphia gladius) was highly migratory oceanic pelagic fish and had a higheconomic value, mainly captured by longline and other fishing gears, the capture of the swordfishin the different depths was selective by longline, Therefore, understanding the characteristics ofthe distribution of swordfish habitat depth could provide a scientific basis for researchingswordfish habitat,reducing bycatch rates, conservation of swordfish resource，improving longlinefishing gear, and so on. Meanwhile, The current fishery resources was in the state of overfishing inthe global，due to the annual strong fishing intensity, no doubt it caused major damage to thefishery resources of the swordfish. So it was very important to master swordfish resource, fishing mortality, recruitment and development status， it could provided a basis to enhance theconservation for the swordfish using quota system and determined the reasonable methods offishing and so on. This study had a positive role in promoting the status of our country in theinternational tuna fisheries research, reflecting a responsible international image of our country inprotecting fishery resources. The main contexts and results in the thesis are as follow:1Characteristics of the habitat depth for swordfish in the central Atlantic Ocean1.1The changes of catch proportion of swordfish in different depthsThe depth range of the captured swordfish was from124.6to280.5m, with an average of215.2m and standard deviation of46.5m.The catch proportion of swordfish was changedsignificantly in the different hook depths, With the increase of hooks depths, the catch proportionof swordfish in total increased to a maximum value at the depth of220m to250m and thendecreased. The maximum of catch proportion in trip1was in the depth of234.1m and250.3m,21%and17%respectively,The maximum of catch proportion in trip2was in the depth of249.4m、226.9m and199.6m,19%、16%and16%respectively, so they were the highest at depth rangeof220~250m, This depth was likely to be the swordfish most active water layer, below the depthof250m，with depth increase, the catch proportion of swordfish showed a downward trend, wecould see from the two cruises, the catch proportion was the smallest in the deepest hooks, only9%and4%respectively.1.2The changes of sex ratio of swordfish in different depthsAt the shallower depths，the proportion of male swordfish was larger，the proportion offemale swordfish in catch increased with depths of hooks and then remained stable. At theshallowest depth（120~150m），the sex ratio of swordfish was least and didn’t accord with1:1，they accorded with1:1in the other depth，The sex ratio was largest and the value was1.364inthe depth of180-210m，it indicated the proportion of female swordfish was largest in the depthrange，in the other depths，the sex ratio accorded with1:1，in the depth of180-300m，femaleswordfish was larger than male a little1.3The changes of length size of swordfish in different depthsThe mean fork length of swordfish tended to increase with depths of hooks. The mean lengthof hooked swordfish was the least at150~180m and120to150m,137cm and140.2cmrespectively. The mean length of hooking swordfish was the largest at270~300m, it was162.1cm. The difference test on the mean length of swordfish in different depth showed that the mean length of swordfish was significant differences at the hook depth of120~150m and270~300m. The mean length of swordfish was a significant difference at the hook depth of150~180m,270~300m and210~240m. The length between any two of the remaining mean depth rangewas not significant difference. This showed that the smaller individuals of swordfish was in themaximum proportion at120to180m which were in shallower depth, the larger individuals ofswordfish was in the maximum proportion at270~300m which were in deeper depth,210~240m depth took second place.2The status of the resource for swordfish in the north Atlantic Ocean.The results were as follows by age–structure VPA，the biomass of resources for swordfishwas over63thousand tons（age0not included）in the north Atlantic Ocean in Jan-12008.thebiomass of age-1was16.6thousand tons.16.1thousand tons for age-2，13.7thousand tons forage-3.7.4thousand tons for age-4，9.4thousand tons for age-5and other age.The total individualsfor swordfish was over2.45million（age0not included），1.23million for age-1,0.60million forage-2，0.34million for age-3,0.14million for age4，0.14million for age-5and older age. Fishingmortality for age-0was least in2007，the value was0.0041,age-1and age-2were least too，0.0619and0.2353respectively，age3-5were lower，the value was0.3633，the fishing mortality waslower to0.25for all ages，the mean biomass was47thousand tons in2007,7thousand tons forage-0,94thousand tons for age-1,11.8thousand tons for age-2,7.7thousand tons for age3.5thousand tons for age4,6thousand tons for other ages，the biomass was greater than biomassof themaximum sustainable yield, swordfish had not been overfishing by Jan-12008. Spawning biomassof swordfish were most for age5and older ages, spawning population biomass was at least for age0. If natural mortality and fishing mortality was bigger before the spawning, the spawning stockbiomass was smaller. Catch biomass also showed a tendency to decline from2000to2007, catchbiomass was10000t in2007, below the level in1992-1999.