| The target species of longline are bigeye tuna,yellowfin tuna and swordfish. However, longline fishing is a low-selectivity fishing method which can result in bycatch issues. The bycatch refers the non-target species, the reserved species and the discarded species because of the economic, legal and other reasons. Bycatch mainly includes the larval individual of the target species, sharks, turtles, marine mammals and some other protected species. At present, the statistical coverage of bycatch is still low. Research was based on the data by two scientific observer surveys in high seas of tropical Atlantic Ocean(05°37′N~14°59′N,29°00′W~40°54′W)by the tuna longline fishery targeting bigeye tuna during 2007~2010. The biological information of main bycatch species, catch and fishing depth were recorded and analyzed by sampling. The main results of the study were as follows:1. Species of bycatch and catch rate were as follows:Longline fishery caught 34 species include 22 families, fishes include 29 species, marine mammals include 3 species, and sea turtles include 2 species in the Atlantic Ocean. Main species of bycatch by catch rate from high to low are as follows: blue shark, yellowfin tuna, swordfish, crocodile shark, dagger pomfret, albacore, bigscale pomfret, escolar, blue marlin, longbill spearfish and bigeye thresher shark.2. Biology of main species of bycatch was as follows:The average fork length of yellowfin tuna amounted to 134.6cm (s.d. =20.1, n=231), the relationship between fork length and dressed weight is: DW=5.72x10-6FL3.1841(R2=0.9614)The sex ratio of female and male is 1:1. When the fork length is more than 170cm, the sex ratio of male is 100%. The average ovary gonosomatic index (GSI) is 0.73(s.d. =0.34, n=45), the average testis gonosomatic index is 0.40(s.d. =0.40, n=40). The average fork length of albacore amounted to 108.2cm (s.d. =6.3, n=83), the relationship between fork length and total weight is: TW=3.73x10-5FL2.8559(R2=0.8277)The average low jaw fork length of swordfish amounted to 148.3cm (s.d.=44.5,n=156), the average low jaw fork length of female swordfish amounted to 182.7cm(s.d.=35.6,n=62),The average low jaw fork length of male swordfish amounted to 136.7cm(s.d.=18.6,n=34),The relationship between low jaw fork length and dressed weight is: Female: DWF=7.41x10-7LJFL3.4798(R2=0.9688) Male: DWM=2.08x10-6LJFL3.2828(R2=0.8842)The sex ratio of female and male is not 1:1, when the low jaw fork length is more than 180cm; the sex ratio of female is 100%. The average ovary GSI is 0.56(s.d. =0.35, n=38), the average testis GSI is 0.21(s.d. =0.21, n=15).The average low jaw fork length of longbill spearfish amounted to 175.4cm (s.d.=10.9,n=41),The average low jaw fork length of female longbill spearfish amounted to 175.3cm(s.d.=6.3,n=12),The average low jaw fork length of male longbill spearfish amounted to 175.4cm(s.d.=11.1,n=16),The relationship between low jaw fork length and total weight is: TW=5.92×10-6LJFL2.9292(R2=0.7004)The sex ratio of female and male is 1:1, when the low jaw fork length is less than 165cm or more than 190cm, the sex ratio of male is 100%. The average ovary GSI is 1.11(s.d. =0.99, n=12), the average testis GSI is 0.16(s.d. =0.16, n=16). The average fork length of escolar amounted to 111.8cm (s.d.=21.3,n=51),The average fork length of female escolar amounted to 121.1cm(s.d.=15.8,n=15),The average fork length of male escolar amounted to 105.4cm(s.d.=24.3,n=17),The relationship between fork length and total weight is: TW=1.06x10-5FL3.0272(R2=0.9823)The sex ratio of female and male is 1:1, when the fork length is less than 80cm; the sex ratio of male is 100%. When the fork length is more than 140cm, the sex ratio of female is 100%. The average ovary GSI is 14.46(s.d. =5.20, n=14), the average testis GSI is 2.62(s.d. =2.21, n=15).The average fork length of blue shark amounted to 211.3cm (s.d.=20.2,n=306),The average fork length of female blue shark amounted to 207.4cm(s.d.=17.2,n=147),The average fork length of male blue shark amounted to 214.9cm(s.d.=22.1,n=159),The relationship between fork length and total weight is: TW=1.62x10-5FL2.8432(R2=0.8376) Female: TWF=0.0004FL2.2373(R2=0.7873) Male: TWM=1.19x10-6FL3.3201(R2=0.9329)The sex ratio of female and male is 1:1. The average weight of ovary is 133.5g (s.d.=98.3,n=49), the average weight of total gonad is 394.8g(s.d.=166.8,n=49).The average weight of testis is 513.6g(s.d.=207.0,n=85), the average weight of total gonad is 708.3g(s.d.=242.2,n=85).23 sea turtles were caught; include 21 leatherback turtles and 2 olive ridley turtles. The average shell length of leatherback turtle amounted to 94.9cm, the average total weight of leatherback turtle amounted to 75.0kg, the average shell length of olive ridley turtle amounted to 50cm, the average total weight of olive ridley turtle amounted to 19.5kg. The relationship between fork length and total weight of leatherback turtle is: W=4×10-5SL3.1507(R=0.9912,n=21).3. Vertical distribution structure of main bycatch species is as follows: Except dagger pomfret, escolar and sea turtle, depth distribution of the rest of the species all accord with normal distribution. Except swordfish, mean depth of the rest of the species are all significantly different with bigeye tuna. And except swordfish and dagger pomfret, depth distributions of the rest of the species are all significantly different with bigeye tuna. Cluster analysis showed that there are three vertical distribution structures. The first kind is the most shallow depth distribution which include 2 kinds of sea turtles, the second kind is the deeper depth distribution which include dagger pomfret, blue shark, blue marlin, longbill spearfish, albacore, yellowfin tuna, crocodile shark and escolar, the third kind is the deepest depth distribution which include bigscale pomfret, bigeye tuna, swordfish and bigeye thresher shark. There are not significantly different in depth distribution between females and males of bigeye tuna, yellowfin tuna, swordfish, blue shark and crocodile shark. Correlation of depth and individual length of these species are also not significant. |
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