TROPHIC RELATIONSHIPS BETWEEN PREDATORS CAUGHT BY TUNA PURSE-SEINERS SEINERS IN THE EASTERN PACIFIC OCEAN DR. FELIPE GALVAN- MAGAÑA CENTRO INTERDISCIPLINARIO DE CIENCIAS MARINAS. LA PAZ, BAJA CALIFORNIA SUR, MEXICO DR. ROBERT J. OLSON INTERAMERICAN TROPICAL TUNA COMMISSION LA JOLLA CALIFORNIA
30 25 Area A 20 15 10 Area C LATITUDE (DEG.) 5 0-5 Area B Area D -10 Area F Dolphin Sets -15 School Sets Area E Floating Object Sets -20-140 -130-120 -110-100 -90-80 LONGITUDE (DEG.)
Table. Stomachs sampled from predators in the bycatch of tuna purse-seiners in the Eastern Pacific Ocean during 1992 1994. Predator Samples Number Stenella attenuata Spotted dolphin 311 Stenella longirostris Spinner dolphin 209 Delphinus delphis Common dolphin 51 Stenella coeruleoalba Striped dolphin 5 Thunnus albacares Yellowfin tuna 4831 Katsuwonus pelamis Skipjack 1205 Thunnus obesus Bigeye Tuna 80 Euthynnus lineatus Black skipjack 100 Auxis spp. Bullets 55 Carcharhinus limbatus Black tip shark 262 Carcharhinus falciformis Silky shark 64 Carcharhinus longimanus W hitetip shark 30 Carcharhinus leucas Bull shark 2 Carcharhinus spp. Carcharhinid sharks 82 Sphyrna spp. Hammerhead sharks 48 Isurus oxyrinchus Mako shark 4 Prionace glauca Blue shark 2 Alopias spp. Thresher shark 12 Nasolamia velox W hite nose shark 2 Makaira indica Black marlin 25 Makaira mazara Blue marlin 15 Makaira spp. Marlins unidentified 18 Tetrapturus audax Striped marlin 8 Istiophorus platypterus Sailfish 49 Coryphaena hippurus Mahi-mahi 545 Acanthocybium solandri W ahoo 235 Elagatis bippinulata Rainbow-runner 48 Total 8298
Predator and prey in dolphins sets 50% 40% Weight percentage 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Mesopelagic Fishes Myctophidae Phosichthyidae Gempylidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Wahoo Black Skipjack Big Eye Skipjack Yellowfin Auxis spp. Sharks 36 (31) 14,157g Billfishes 9 (2) 1,615g Other species 4 (3) 418g Dolphins 389 (187) 18,133g Tunas 1579 (505) 336,128g
Predators and prey in school sets 70% 60% Weight percentage 50% 40% 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Myctophidae Phosichthyidae Gempylidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Wahoo Black skipjack Skipjack Yellowfin tuna Auxis spp. Sharks 124 (26) 99,577 g Billfishes 52 (10) 17,004 g Other species 23 (22) 5,483 g Tunas 1242 (979) 126,327 g
Predators and prey in Log sets 60% 50% Weight percentage 40% 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Mesopelagic Fishes Myctophidae Phosichthyidae Gempylidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Wahoo Black skipjack Big eye Skipjack Yellowfin tuna Auxis spp. Sharks 233 (58) 292,660 g Billfishes 27 (15) 23,533 g Other species 389 (388) 49,913 g Tunas 261 (1706) 12,643g
TUNAS
Tunas 100% 90% Weight percentage 80% 70% 60% 50% 40% 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Myctophidae Phosichthyidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Black Skipjack Skipjack Yellowfin Tuna Auxis spp. Yellowfin Tuna 2,895 (1,936) 464,891g Skipjack135 (1,070) 7,939g Bullets 2(53) 8g Black Skipjack 12 (88) 577g Big eye 38 (42) 1,638g
DOLPHINS
DOLPHINS 100% 90% 80% Weight percentage 70% 60% 50% 40% 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc. Fishes Myctophidae Phosichthyidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Skipjack Auxis spp. Striped dolphin 5 (0) 125g Spotted dolphin 231 (80) 12,080g Spinner Dolphin 139 (70) 5,611g Common dolphin 14 (37) 317g
SHARKS
Sharks 100% 90% 80% Weight percentage 70% 60% 50% 40% 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc. Fishes Myctophidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Wahoo Black Skipjack Big eye Skipjack Yellowfin tuna Auxis spp. Hammerhead sharks 44(4) 31,238g Bull shark 2(0) 410g Blue shark 1(1) 125g White nose shark 2(0) 6,660g Carcharhinids Sharks 57 (25) 44,368g Thresher shark 9(3) 6,653g Silky Shark 51 (13) 61,444g White tip shark 15(15) 18,661g Mako Shark 3(1) 3,621g Black tip shark 209 (53) 233,214g
BILLFISHES
Billfishes 80% 70% Weight percentage 60% 50% 40% 30% 20% 10% Unidentified Marlins 16(2) 9,134g Striped marlin 5(3) 7,082g 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Myctophidae Phosichthyidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Skipjack Yellowfin tuna Auxis spp. Sailfish 43(6) 5,166g Blue marlin 9(6) 11, 563g Black marlin 15 (10) 9,207g
OTHER PREDATORS
Other predators 50% 40% Weight percentage 30% 20% 10% 0% Epi Cephalopods Meso Cephalopods Crustacea Misc.Fishes Myctophidae Phosichthyidae Carangidae Coryphaenidae Exocoetidae Nomeidae Tetraodontiformes Wahoo Black skipjack Big eye Skipjack Yellowfin tuna Auxis spp. Wahoo 113(122) 21, 325g Rainbow runner 32(16) 2,834g Mahi-Mahi 271(274) 34,805g
30 25 Area A 20 15 10 Area C LATITUDE (DEG.) 5 0-5 Area B Area D -10 Area F Dolphin Sets -15 School Sets Area E Floating Object Sets -20-140 -130-120 -110-100 -90-80 LONGITUDE (DEG.)
Table. Overlapping between predators in the Eastern Pacific Ocean. T=Tuna, D=Dolphin, S=Shark, B=Billfishes, O= Other predators (Mahi-mahi, wahoo, rainbow runner) Numbers in bold indicate values higer than 0.6, which could indicate overlapp between predators Area/set type T-D T-S T-B T-0 D-S D-B D-O S-B S-O B-O Area A/Dolphin set 0.43 0.04 0.2 0.27 0.17 0.18 0.28 0.02 0 0.02 Area B/Dolphin set 0.41 0.26 0.39 Area C/Dolphin set 0.24 0.11 0.77 0.75 0.18 0.16 0.11 0.05 0 0.95 Area D/Dolphin set 0.29 0.52 0.55 Area E/Dolphin set 0.47 Area F/Dolphin set 0.50 0 0.01 Area A/School set 0.67 0.48 0.24 0.44 0.1 0.41 Area B/School set 0 0 0 Area C/School set 0.39 0.23 0.15 Area D/School set 0.16 0.03 0.21 0.42 0.14 0.12 Area E/School set 0 0 0.35 0.28 0 Area F/School set 0.01 0.99 0 Area B/Log set 0.02 0.1 0.33 0.6 0.45 0.51 Area C/Log set 0.69 Area D/Log set 0.11 0.11 0.49 0.55 0.27 0.29 Area E/Log set 0.18 0.12 0.43 0.46 0.3 0.45 Area F/Log set 0.08 0 0.44 0.75 0.56 0.46
Tabla 2. Breadth niche recorded in predators by each area in the EPO. D=Dolphins sets, S= School sets, L= Log sets Area A Area B Area C D S L D S L D S L TUNAS 2.8 7.0 3.4 1.0 2.5 1.8 2.3 1.8 DOLPHINS 6.4 6.2 5.1 OTHER PREDATORS 3.0 2.0 4.9 1.0 3.0 BILLFISHES 1.7 6.4 1.0 1.8 1.0 1.9 SHARKS 1.4 3.9 3.0 1.0 4.1 1.6 3.1 Area D Area E Area F D S L D S L D S L TUNAS 1.7 2.1 8.9 1.1 1.1 1.7 1.3 1.0 1.2 DOLPHINS 6.1 17.6 4.5 OTHER PREDATORS 2.2 11.9 2.0 6.3 1.4 1.0 4.7 BILLFISHES 2.2 5.3 1.0 2.0 2.0 SHARKS 3.2 3.3 2.6 2.7 5.1 1.6 2.9
100% Dolphin Sets Prey Biomass 90% 80% 70% 60% 50% 40% 30% 20% 10% Misc. fishes Crustacea Cephalopoda Myctophidae Carangidae Tetraodontiformes Exocoetidae Phosichthyidae Nomeidae other Scombridae Auxis spp. 0% 300-600 601-800 801-1000 1001-1200 1201-1400 1401-1889 72 227 344 556 254 108 Fork Length (mm) Sample Size
100% Schoolfish Sets Prey Biomass 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 300-600 601-800 801-1000 1001-1200 1201-1400 1401-1889 237 392 258 123 84 22 Fork Length (mm) Sample Size Misc. fishes Crustácea Cephalopoda Myctophidae Carangidae Tetraodontiformes Exocoetidae Phosichthyidae Nomeidae other Scombridae Auxis spp.
100% Log Sets Prey Biomass 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 300-600 601-800 801-1000 1001-1200 1201-1400 135 24 19 9 6 Fork Length (mm) Sample Size Misc. fishes Crustacea Cephalopoda Myctophidae Carangidae Tetraodontiformes Exocoetidae Nomeidae other Scombridae Auxis spp.