MFR PAPER 1272 The authors are with the Southwest Center, La Jolla Labora­ tory, National Marine Fisheries Ser­ vice, NOAA, La Jolla, CA 92038.

A Review of the Yellowfin-Skipjack trated primarily off Africa (Fig. 2). of the Atlantic and Some areas in the western Atlantic, such as in the Sea and off American Participation, 1956-75 Venezuela, support a small surface fishery that is probably underdeveloped (J uhI, 197I; Ramos and Guerra, 1976; Wagner, 1974). In this report we review the de­ GARY T. SAKAGAWA, ATiLlO L. eOAN, and TED e. MURPHY velopment ofthe tropical tuna fishery of the Atlantic from 1956 to 1975, and discuss the current condition of the stocks. Special attention is given to analysis of data from the American! INTRODUCTION 240 tropical tuna fleet because this fleet is a [\ Over the past 2 decades the tropical 200 TOTALi \ relative newcomer to the fishery but i \ tuna, or combined , 160 / ranks fourth in total production. Data / ...... _./ sources for this study are: ICCAT albacares, and , 120 i Katsuwonus pelamis, fishery of the At­ / ...... /0.-··- j vELLOWFIN I\ (1976a) for data on catch and number of 80 ,/ ',,--' \ lantic Ocean has increased from a few SKIPJACK \ hundred metric tons in 1956 to approx­ 40 I The combined tropical tuna fleets of Canada, imately 174,000 metric tons in 1975 Netherlands Antilles, Panama, and the United 1960 1970 .975 (Fig. I). It is currently the largest tuna YEAR States are monitored as a unit. "American" in fishery in the Atlantic Ocean and sup­ this report refers to this combined fleet, which Figure I.-Total Atlantic catches of yel­ during 1968-75 consisted of at least 83 percent ports an international fleet of more than lowfin and skipjack . U.S. vessels. 800 vessels. The vessels employed in the fishery Figure 2.-Areas in the Atlantic Ocean where the longline fishery and surface fishery (speckled are mainly longliners, baitboats, and area in the eastern Atlantic) for tropical tunas are concentrated. purse seiners. They range in size from a few metric tons to 2,000 metric tons in 60·W carrying capacity. Longliners generally fish well offshore with longline gear (Shapiro, 1950) and catch deep­ swimming tunas of frequently large size. Baitboats employ the pole-and­ line with live-bait chum technique (Cleaver and Shimada, 1950), and purse seiners employ purse seine nets (McNeely, 1961). Both baitboats and purse seiners operate relatively close O· inshore and purse seiners also operate far offshore. They capture tunas, fre­ quently of small size, that occur near the surface. Because of this difference in the type of tunas exploited by the different gears, the fishery is often di­ vided for convenience into a longline fishery, consisting of longliners, and a surface fishery, consisting of baitboats and purse seiners. The longline fishery extends '"------150• 5. throughout the tropical Atlantic, 60·W. 30·W. O· 30·E. whereas the surface fishery is concen-

December 1977 1 vessels; Fishery Agency of shifted its operations to temperate re­ Japan (1968-75) for data on longline 500 gions in search of other species such as catch and effort; and ICCAT (l973a, b, and (Shiohama et 1974a, b, 1975a, b) for data on sizes of aI., 1965). The species mixture of the fish caught and catch and effort of the '00 catch, consequently, changed from surface fleets. primarily yellowfin tuna to primarily temperate tunas. Yellowfin tuna pres­ LONGLINE FISHERY ently contributes less than 30 percent The longline gear is particularly ef­ of the total longline tuna catch and al­ fective in catching pelagic fishes that bacore and bigeye tuna constitute more are widely dispersed in the open ocean. than 60 percent of the catch. It is a multispecies gear but can be de­ ployed to catch more of one species Yellowfin Tuna Catch than another. In longline tuna fishing, and Catch Rates albacore (T. alalunga), bigeye (T. Longliners were responsible for most obesus), bluefin (T. thynnus and T. of the catch of Atlantic yellowfin tuna maccoyii), and yelJowfin tuna are before 1966 (Fig. 5); since then the 1960 ~, /970 1973 primarily caught along with incidental YEAR surface fishery has produced most of catches of (Istiophoridae and Figure 3.-Number of vessels in the the yellowfin tuna. The longline catch Xiphiidae). Skipjack tuna is normally long line fishery of the Atlantic Ocean, peaked at about 53,000 metric tons in not caught on longline gear. 1956-73. 1960 and slowly declined to 21,000 for tunas in the metric tons in 1967. It then increased to oceanic regions of the Atlantic Ocean about 30,000 metric tons in 1969 where was pioneered by the Japanese in 1956. it has remained. In that year a few Japanese longliners More than 50 percent of the longline conducted test fishing for tunas off the catch of yellowfin tuna was annually northeastern coast of South America eo Yl.....LOWflN caught in the eastern Atlantic (east of (Shiohama et al., 1965). Fishing was long. 30 0 W) during 1957-64 (Fig. 6). excellent particularly for yellowfin GO About equal amounts were caught from tuna, and in the following year the fish­ the eastern and western Atlantic during ery began in earnest with 26 Japanese 1965-69 and the bulk from the western longliners fishing throughout the tropi­ Atlantic (west of long. 30 0 W) during cal Atlantic. Additional Japanese ves­ 1970-75. sels entered the fishery each year there­ Ifthere are two principal subpopula­ after until the Japanese fleet peaked at tions (Honma and Hisada, 1971; 2 about 210 vessels in 1965. Hayasi ), eastern and western subpopu­ The catch rates for the Japanese fleet lations, ofyellowfin tuna in the Atlantic were initially high but quickly declined Ocean, the longline catch rates suggest Figure 4.-Species mixture in the long­ as the fleet rapidly increased in size. line catch (tonnage) of the Atlantic that the eastern subpopulation is more This caused some vessels to withdraw Ocean. 1957-75. depressed than the western subpopula­ from the fishery and some to be sold to tion (Fig. 7). The catch rate in the east­ foreign interests, primarily to South ern Atlantic fell sharply from about 3.6 Korean and Taiwanese fishermen. A terests, about 40 percent to Japanese metric tons/l,OOO hooks in 1958 to few new, more efficient Japanese ves­ interests, and the remainder to Brazi­ about 0.8 metric ton/l,OOO hooks in sels entered the fishery as replacements lian, Cuban, Panamanian, and Ven­ 1962, and then gradually declined to while the former Japanese-owned ves­ ezuelan interests. less than 0.1 metric ton/l ,000 hooks in sels were placed into service by the new 1975. In the western Atlantic it fell pre­ owners. The Koreans and Taiwanese Species Mixture cipitously from about 3.3 metric tons/ found fishing in the Atlantic to be Before the mid-1960's, longlining in 1,000 hooks in 1958 to about 0.7 metric profitable and quickly acquired and de­ the Atlantic was primarily confined to ton/l,OOO hooks in 1961 and then ployed more vessels. Consequently, tropical waters where the yellowfin drifted lower to 0.4 metric ton/l,OOO the international longline fleet grew tuna occur. Yellowfin tuna was the hooks in 1975. rapidly in the late 1960's and early target species and contributed more 1970's (Fig. 3). Currently, more than than 30 percent of the total longline 500 longliners participate in the Atlan­ tuna catch (Fig. 4). The catch rate of 2Hayasi. S. 1973. A hypothesis on population structure of yellowfin tuna in the Atlantic Ocean. tic tuna fishery with about 50 percent yellowfin tuna, however, declined dur­ mainly based on longJine data. ICCAT Collective belonging to Korean and Taiwanese in- ing this period and the fleet gradually Vol. Sci. Pap., 2 (SCRS-1973), p. 40-48.

2 Marine Fisheries Review 120 TOTAL / ..' . Vi .1' _.- ...... -g100 o ././ EASTER "'-"'.",. .-._.---.. - If) g i ...... / / ,-- ,--- -, , £: 80 ___ I ,_.-, / .~ / -",/WESTERN If) c of--r--l---,---,-.-.---r--r-r-r--r,.-..,--,--,-.,..,..., 2 60 1957 1960 1965 1970 1975 () ',: YEAR ~ 40 Figure 6.-Longline catches of yellowfin tuna from the eastern and western Atlantic, 1957·75. I ------U ~ SURFACE U

Figure 5.-Atlantic catches of yellowfin tuna for the 1960 1965 1970 1975 longline and surface fisheries, 1957·75. YEAR

10 10 ;:: NUMBERS '"--< VJ B 4 ;0 VJ 8 0'" -', <> 0 0 , , EASTERN ATLANTIC 0'" I , --< WESTERN ATLANTIC , 0 I -. \ 6 3 z \ 0 , (/) 0 \ \ Q .... Q \ .... TONS\ .... \ a:: 4 \ 2 '0 a:: 4 2 "0 w \ 0 W TON's" o lD \ 0 lD o ::!' \ ::!' \ ::::> \ I \ I 2 I 0 ::::> \ 1 0 Z \ Z 2 \ --,, 0 ...... a , :>< - :>< ", (/) - "- _------(/) 0 0 0 ----- .. .--- 0 1957 1960 1965 1970 1975 1957 1960 1965 1970 1975

YEAR YEAR Figure 7.-Catch rates of yellowfin tuna for the Japanese longline fleet by eastern and western Atlantic.

Size Composition of Figure S.-Average fork length of yeIlowfin tuna caught in the longline YeUowfin Tuna fishery of the eastern and western Atlantic. Before 1970, yellowfin tuna caught in the eastern and western Atlantic by 200 the longline fishery were, on the aver­ age, about the same size (Fig. 8). Since E 150 EASTERN the, the average size of fish caught in 2 ------/ the western Atlantic has consistently I ,----'" -----_// t- been smaller. The average size of fish ~ 100 WESTERN caught in the eastern Atlantic further­ w -.J more has remained quite constant de­ :x: 0: spite the competition from the surface ~ 50 fishery that lands substantial numbers of large yellowfin tuna (>85 em, Fig. 9). These data suggest the possibility Ol--,--.,--,.-.....,.---.----,,.-.....,.--,.----r--,--.---, that the longline and surface fisheries 1963 1965 1970 1975 do not exploit the same stock (Lenarz YEAR and Zweifel, in press) and that in the western Atlantic the longline fishery is exploiting greater amounts ofyellowfin SURFACE FISHERY eastern and western Atlantic. The prin­ tuna from the surface stock than the Yellowfin and skipjack tunas are cipal fishing areas are within 200 miles deep-water stock. caught with surface gears in both the of the coast and currently most of the

December 1977 3 Figure 9.-Estimated length composition of yellowfin tuna caught in the longline and lantic fishery and their total catch was surface fisheries of the Atlantic Ocean in 1974. 13,960 metric tons of yellowfin tuna 80 and 7,370 metric tons of skipjack tuna. LONGLINE FISHERY In the western Atlantic, the surface 60 fishery for tropical tunas is small and 40 not well developed. A few thousand

20 metric tons are produced annually, principally by Cuban and Japanese baitboats, and French, Spanish, and American seiners. Cuban baitboats

600 have fished primarily skipjack tuna in SURF ACE FISHERY the Caribbean since 1932 (Suarez­ 400 Caabro and Duarte-Bello, 1961),

~ 200 whereas the other fleets are more recent ~ :10 participants and only occasionally EASTERN _ 80 fished in the western Atlantic for tropi­

V> cal tunas. American seiners of the trop­ 0: W 60 al ical tuna fleet normally fish in the west­ :l; :::> z 40 ern Atlantic only while crossing the region enroute to Puerto Rico (to un­ 20 WESTERN load their catch) or fishing grounds in o f--f--,------,--,..---r"'-----,'-----,.----,'--''--::',--'""--,---;;-.,--,-..,..-,-:"=j==-,----, the eastern Atlantic and eastern Pacific. 20 30 40 50 60 70 80 90 100 110 These vessels, therefore, do not spend LENGTH (em.l many days searching for tunas in the western Atlantic. Because the surface fishery of the western Atlantic is small, this report does not attempt to fully dis­ Atlantic catch is caught in the eastern Teble 1.-Number of AmerIcan tune pur..e.l.....e' end cuss this fishery. their catches from the eeetem tropIcal Atlentlc, 1967-75. Atlantic off Africa. Catch rate Species Mixture The eastern Atlantic surface fishery Catch (I) (Vday's fishing) was an artisanal fishery up to about - Skip- Yellow- Skip- Yellowfin and skipjack tunas are 1955. In that year, French baitboats No. of fin jack fin jack target species for the surface fishery. Year vessels tuna tuna tuna tuna from southern Europe moved into the Yellowfin tuna, however, have tradi­ 1967 3 920 480 7.8 3.8 Dakar, Senegal, region to fish for yel­ 1968 6 5.630 3.160 23.3 12.0 tionally been preferred by tropical tuna 1968 27 19,760 4.690 10.9 2.4 lowfin tuna during November to May, 1970 25 9,810 11,790 4.0 5.1 fishermen because it commands a when the boats were not employed in 1971 24 3,630 16,630 2.7 10.0 higher market price. In fact, until re­ 1972 36 12.100 12,250 3.3 3.7 the albacore fishery off the Iberian 1973 24 3,300 22,290 2.2 17.0 cently some fishermen, particularly peninsula (Champagnat, 1968). The 1974 26 5,620 19,970 2.8 6.7 French fishermen, avoided the capture boats were successful and baitboats of 1975 32 13,960 7,370 5.6 2.7 of skipjack tuna and caught them only 'The combined tropical tuna fleet of Canada, Netherlands other countries soon entered the fish­ Antilles, Panama, and the Unijed States are monitored as a incidentally with yellowfin tuna. unit. "American" refers to this combined fleet which in ery. The fishery quickly expanded into 1968-75 consisted of at least 63 percent U.S. vessels. During the 1960's, when the catch southern waters and by 1960 the fleet rates of yellowfin tuna were high, an was operating year-round from about average of 56 percent of the total Port Etienne, Spanish Sahara, to Pointe the eastern Atlantic fishery (ICCAT, yellowfin-skipjack tuna surface catch Noire, Zaire. 1976a). The largest fleet currently be­ from the eastern Atlantic was yellowfin Purse for tropical tunas, longs to the combined French, Ivory tuna. As more units of fishing effort which was developed by U.S. Pacific Coast, and Senegal (FIS) countries. were applied to the fishery, the catch coast tuna fishermen in 1956 Significant American participation in rates plummeted and the fishermen (McNeely, 1961), was first tested in the the Atlantic fishery began in 1967. In began relying more on skipjack tuna to Atlantic in the late 1950's. The that year, three American tuna purse fill their boats. The average percentage technique was not seriously used in the seiners fished off Africa and caught of yellowfin tuna in the catch declined Atlantic tropical tuna fishery, however, 1,450 metric tons of tuna in little over 2 to 42 percent during 1970-74. until 1961 by the French. Seiners of months of fishing (Sakagawa and Similar events occurred in the other countries soon thereafter entered Lenarz, 1972). Their success attracted American catch (Table I). YeUowfin the fishery. Currently more than 300 other American seiners into the fishery tuna dominated the catch during 1967­ baitboats and purse seiners belonging to (Table I). In 1975, 32 American tuna 69 and skipjack tuna during 1970-74. In 12-15 countries participate annually in seiners participated in the eastern At- 1975, however, skipjack tuna were

4 Marine Fisheries Review / Figure IO.-YeUowfin tuna catch in '0 x 1° y areas of the eastern tropical Atlantic. Log­ I) METRIC TONS book catches from the combined French, • >1000 Ivory Coast, Senegalese, and American fleets 00 for 1975 are shown. 000 1m 500-999 0 •• ~ IZI 2~0-499 00 ~ 181 100-249 lZl 50 - 99 Ii) 1- 49 IQJ EFFORT. No Cotch

o o o o o

YELLOWFIN TUNA o o o 0 0 0 •• Luanda o o 000 ooeo - o o .~ o o 00 A -J-+-.j.....i-l--l-I~+-I-I--+-+-+~ol-l-++-+-+-+-1~o~o:d-+-1""oH++-1""'oH++-t""'oHo~of.o~lqT<9o 000. .. o o • "Pc..: 00 o o o .0 (01& o

o 00

o

1\

\ II.. r II SKIPJACK TUNA

Figure 12.-Percentage of 1974 Atlantic catch of yellowfin tuna -;;; 100 and skipjack tuna landed by major '0 fleets. FIS fleet refers to French, c: 0 Ivory Coast, and Senegalese

December 1977 5 3 were estimated by Coan and Fox . are based on data from the FIS fleet and other of low catch rates are also evi­ Their estimates for the surface fishery for the series of years 1964-75 (Fig. denced in the time series (Fig. 13). The 13). The series of estimated catch per period of high catch rates was during 3Coan, A. L., and W. W. Fox. 1976. A produc­ standard day at sea (SDA) show a peak 1964-69 when the rates averaged 3.6 tion model analysis of Ihe status of yellowfin luna of 4.7 metric tons/SDA in 1968, and a metric tons/SDA. Lower catch rates, in the Atlantic Ocean, 1964-1975. NMFS, Southwest Fisheries Center, La Jolla, Calif., low of2.1 metric tons/SDA in 1973 and averaging 2.3 metric tons/SDA, occur­ Admin. Rep. No. LJ-76-24, 16 p. 1975. Two periods, one of high and the ring during 1970-75. Similar periods can be identified in Figure 13.-Catch the nominal catch rates of the American c rates of yellowfin ~ 5 500, fleet (Fig. 13). Catch rates were well tuna caught in the .S o .<: surface fishery of above 6.0 metric tons/day's fishing dur­ >- Vl .g 4 40 ;;: the eastern Atlan· ing 1967-69 and less than 6.0 metric tic. Catch rates for FLEET _Vl tons/day's fishing during 1970-75. In "E >­ c c the entire surface both of these periods the American -g 3 30 ~ fleel (in metric c Vl tons/standard day catch was made primarily in the third c:: 1ii at sea) and the "-Vl I' !? and fourth quarters of the year, and the c:: 2 1 ' 20 u American fleet (in 1 \ .;: highest yellowfin tuna catch rates were .2 Q; metric Ions/day's u / \ .;: 1 , E fishing) are shown. in the Gulf of Guinea (Figs. 14, 15). Q; E / ", AMERICAN FLEET 10 I ', -- ,..-/ U G OL----I_--l._--l._....J.._-'-_...J..._-'--_.J-_..l-_L-_WO 5 ~ 1964 1966 1968 1970 1972 1974 Figure 15.-Average catch rates of yellowfin and o YEAR skipjack tunas in 50 x 5° areas by quarter caught by American seiners in 1970-75. ..--.--,--,-----,------:-:::::-::---, "" Figure 14.-Average catch rates of yellowfin -I----{ QUARTER 1 QUARTER 2 ~44o\'..Q~\o' 's January - March ~N4RYIS. and skipjack tunas in 5° x 5° areas by quarter 'i--+--J---}'1/ April - June caught by American seiners in 1967-69. (6. BLANC M'elric lons/doys fishing G. SLANG 1--+--1--1 ,--, '0- . _i_reo VEROE IS· YELLOWFIN i-rC.VEROE IS. AFRICA -J...... / QUARTER 3 .- SKIPJACK 11 Ctl"'4.qY1S "\ Daka~os AFRICA '-----' - Dakar ,..1-+-+-/-1 July - September 0 2.~ { .e\,)f\s. .1--+---+_-lC.8LANC Metric Ions/day's fishing 0 127 ~ o 3.0 2.1 ~ 0 0 0 0.5 0 0 0 ---l.G.",,·" "'AFRICA VEU:OWFIN ~~'6 '''I-+-I-~DtlKAR S~K .-9- _0 1.-8- 0 0 ~O- ~:::-,,,, 1.5 0.3 2.!l' 0 1.4 0":;\".>' 0 0 3'~1<;o'''' 0 0 0 3.5 0.1 ~\~~(,: o 0 0 18.9 3.7 0 3.7 0'Q," 0 Q 0 3.1 1.3 0.4 7.7 • f------'-fu~,,,-"'0--r3.....9>r"'v -g-~ NH--;;~i.~H_i~'hl¥~T 0" _0 g g 0" 0 q.b .0

0_ _ _ _lj 123.9. 71 0 _,.y~ 16.8 5.6 11.4 .,.~ o 0 0 0 0 2.9 08 "0 0 5.6 14.8 o .- boo 7.5 0 3.6 0 0 0 0 0.3 0.5 ·[-+---+-1-,0 IA 28 5..2-7~·2,," ~l I I 8.~ ,,50, \f9 .. o. ..0- 0_ 0 6.1 :.~ _Q... 0 0 0 1.4 0 0 g 0 0 0 89 8.9 0 S. I s. "1--.. -I-'L -0Q... 0 6.3 0 0 0 ?~ ~ ~fl o 0 ~.4 .._. .. --I--j--+--+-"-I~ ~ f--- _ _ _ ..Q... . 0 0 13.2 \

-1-- - \% ---:=-1\ 2~~h-~,;\;,,....--,!"\;-,.-+.,,.,....-.,J~;;-. \ -i-,.--;!O":;;--+'s'-E-..,J,o;;-'_;!;;".,--J ;.,. s· 0 s· I--.-.-f---- '[\' ". ",. ". "'. o E. "'. ".

Z5o~L...-,..,,':-.-""0"':---,,':-.---,0"':----=,':-.---':-.---':---=,-O:o.---,~,..,--' 7.1~~ QUARTER 4 i~N4RY QUARTER 3 o C.4~.4~y '--- IS. July - September "" 0 ~(c October - December o C.BLANC. ~4.q1'IS. QUARTER 4 C. BLANC 0 I--+--+--Y OcTober - December O.~) AFRICA 0 0 0 AFRICA 3.4 0 0 10 6) 0 0 2.6 0 0 0 0 .1..8' 0 0 0.9't 0 0 0.3 ~.~ 9.8 5.0 4.6 7.0 3.7 2.9, 2.3 0.1 0.6 5.6 3.0 7.4 4.2 2.2 ~:::-''6 17 3.3 3.5 7.8 0.5 1.6 .j.~,'l. 10° 0 0 o 0 0 0 2.5 ·0 'r%- -%- 1.6 4.4 0.8 2.0 2.4 ,'" 0 2.7 T:4 0.5 2.8 2.1 0'Q(,e o 0 o 0' 0 1.3 0.4 ,~ ~~ ~4" 0 0 0 0 28 4.4 33 3.0 0 2.5 9.9 2.2 1.0 1.3 o 0 o 0 4:6 12.8 11.0 4.9 ,•• o° 0 0 0 0.3 7.8 5.2 10.9 0 0 0 0 4.9 10.8 6.3 ' 4.5 0 o 0 0.7 n2 I 3 q , o 0 0 0.1 2.5 3.8 f'5.3 0 0 0 0 3.3 09 1.9 5.2 s· 0 0 0 9.0 0.8 3.2 0 0 0 2.8 0.4 4.1 I 0 0 7.8 2 ? ,,1, 0 0 0 5.9 0.3 4.5 0'--- 0 0 (j 1.0 I.~ 0 0 0 0 0 o.~ s. 0 122 7 l!>·f-- _ .. 0 0 0 6.6 17.2 ,. 0 0 0 0 0.5 7.9 0 0 0.1 \ 0 157 -0 .?~ ". ,. 0 ,. \ ". 20" ". "'. ,. 0 s· oc· 20" ". "" o E. ". w. o° E.

6 Marine Fisheries Review Yellowfin Tuna made in the Gulf of Guinea, off catch of skipjack tuna was made in Size Composition Senegal, and Angola (Fig. 18). In those areas. 1975, 70 percent of the total Atlantic Catch rates for skipjack tuna are Size and age composition of yel­ lowfin tuna in the surface catch of the eastern Atlantic was estimated by 125 120 4 • Coan His estimates show that fish be­ V> tween 31 and 125 cm long, or 1- and 100 "g100 :> 2-year-olds made up the bulk of the E o ~ YELLOWFIN c catch in 1966-75. The dominant age - 80 :I: 75 oS f- V> group in most years was the I-year-old 1000 average rate of increase was about .0. •181 500-999 10,000 metric tons/year with over 90 0 •• 0 I2l 250-499 • 181 100-249 percent of the catch being taken in the 0 '17v. 0 .010 50-99 Dakar 1Zl eastern Atlantic. In 1975, the catch • 00. 1 Iil 1-49 0 0 i suddenly declined to approximately 0 'I I§) EFFORT, No Catch 0.1 t o. i' 59,500 metric tons from a peak in 1974 I ,0 10 • •• 0 .. of about 114,700 metric tons. The 00 00 •• 00 ,l 0 .0 ••• 0'" 0 cause ofthis sharp decline is believed to 00 0 00 O. b" ~ be due to poor availability, particularly 00 00 •• •• of ,," ~ - 0 0 00 00 O. o.l-'TXI .15<. off Angola in 1975. 0 0 00 o.•• 0 • o. ~. Wide fluctuations in the catch have I 0 00 00 00 00. •• •• 17. .0 00 • 0 0 0 00- 000 000 • •• •• 00000. also been reported for other skipjack 00 00 00 .0 •• ••• • 0 0 0 0 00 00 00 00 •• 00 •• • 0 :IX, .--5aa Tame 1 tuna fisheries, and they appear to be due 0 0 0 00 00 0 00 o. o. • • ~ to changes in availability and strength 1 0 00 00 o. 00 00 • 00 O. 00 00 .0 ••• of year classes (Joseph and Calkins, .0 0 0 00 00 .0•• 0 •• 0 00 00 00.0. Pt NOIre , 00 00·a0000. 0 1969; Uchida, 1976). Oceanographic - 0 conditions are currently believed to be a 0 0 0 00 .0. - 0 00 0 00 0 •• major controlling mechanism for the 000 0 •• Luanda 0 0 000 00 •• fluctuations in availability of skipjack 0 0 eli 0 0 •• ./11 tuna (Williams, 1972). 0 00 O. Spanish, FIS, American, and Jap­ 0 00 0 0 0 00. <9" 0 0 ~" anese tuna boats account for 94 per­ 00 0 0 • ~~ cent of the total Atlantic catch of skip­ • jack tuna (Fig. 12). Most of the catch is 0 ~ 0

0 ·Coan, A. L. 1976. Length and age composition of yellowfin tuna from the Atlantic Ocean, 1966-1975. NMFS, Southwest Fisheries Center, La Jolla, Calif., Admin. Rep. No. LJ-76-22, 23 k ,~ p. I'" 10' III

December /977 7 quite variable. They are affected by the 20 ICCAT, J976b). The analysis is usu­ fishing gear, time and area fished, and ally based on catch-and-effort data whether skipjack tuna and not yellowfin from the fishery and relies on several tuna is the principal target species. A assumptions, including: the model is 15 representative catch rate for the fishery applied to a closed, distinct, self­ is therefore difficult to estimate. sustaining population; the concept of Sakagawa and Coan5 calculated stan­ w equilibrium conditions holds; and the a:'::i dardized catch rates for the skipjack :r: 10 age groups being fished remain con­ tuna fishery based on stratified nominal u stant. These assumptions rarely hold u'::i catch rates by vessel type and month, for tuna populations. Results from the and by assuming that catch rates in the analyses are consequently evaluated central part (Annobon) of the Gulf of together with other information to de­ Guinea (Fig. 2) were representative for termine the state of the stock. the entire eastern Atlantic. Their esti­ Annual appraisals of the condition of mates (Fig. 19) range from 1.2 metric of---,--,....---.,--.,..------,-.,..-----,----, Atlantic yellowfin and skipjack tuna tons/standard day's fishing in 1967 to 1967 1969 1971 1973 1975 stocks and fisheries are conducted by 3.0 metric tons/standard day's fishing YEAR the Standing Committee on Research in 1971 and show peaks in 1971 and Figure 19.-Calch rate of skipjack tuna and Statistics of the International 1974. The 1975 estimate is 1.2 metric caught in the surface fishery of the eastern Commission for the Conservation of Atlantic. Catch rates for the entire surface tons/standard day's fishing, or 43 per­ fleet (in metric Ions/standard day's Atlantic Tunas (ICCAT). The apprai­ cent below the estimate for 1974. This fishing) and the American fleet (in melric sals have been based to a large extent on decline is less than the 52 percent de­ tons/day's fishing) are shown. production model analysis. crease observed in the catch (Fig. 17), Yellowfin Tuna indicating that the stocks were not fished as hard as the decline in catch The most recent appraisal of the state suggests. 20 of Atlantic yellowfin tuna stocks was Skipjack tuna catch rates for the made in 1976 (lCCAT6). The appraisal 16 oz showed the stocks to be healthy and the American fleet are quite variable, but w :J catch increasing primarily because of the trends are generally similar to those 8 12 a: noted for the standardized catch rates of "- offshore expansion of the surface Sakagawa and Coan (Fig. 19). Most of ~ 8 fishery and strong recruitment of fish w U the American catch of skipjack tuna 0:: into the fishery. The maximum sustain­ since 1970, however, was made off ~ 4 able average yield (MSAY) was esti­ Angola rather than in the Annobon re­ oi-"O""--,----.,------=;:="""""l---, mated to be 116, 100 metric tons for the gion. The catch rates off Angola were 30 40 50 60 70 80 entire Atlantic fishery and 95,700 met­ exceptionally high in the third and FORK LENGTH (em) ric tons for the eastern Atlantic surface fourth quarters of the year when most Figure 20.-Length frequency distribu­ fishery only (Coan and Fox, footnote fishing by the American fleet took place tion of skipjack tuna caught in the eastern 3). These estimates were derived from a (Fig. 15). High catch rates were also Atlantic in 1974. broad, flat-topped production model recorded in the first and second quarters which indicate that only a negligible in the Gulf of Guinea and off Senegal, (Fig. 20). For the past 5 years, I-year­ increase in equilibrium yield would re­ but they occurred during the year when old fish (30 to 50 cm long) have domi­ sult from further increases in fishing not much fishing by the American fleet nated the catch, accounting for more effort (Fig. 21). This conclusion has took place. Consequently, they are not than 50 percent of the numbers of skip­ also been reached with yield per recruit considered too reliable as indices of jack tuna caught (Coan, 1976). models (Lenarz et aI., 1974). The abundance. The American catch of skipjack tuna model also implies that at infinite has likewise consisted of principally l­ fishing effort MSAY will be sustained Skipjack Tuna and 2-year-old fish, with I-year-olds from a reduced stock. This, of course, Size Composition dominating the catch (Sakagawa et aI., is impossible so conclusions drawn The Atlantic surface catch of skip­ 1976). The average length of fish in the from extrapolation of the equilibrium jack tuna consists primarily of 30- to American catch has fluctuated between yield curve too far to the right should be 60-cm long fish, or 1- and 2-year-olds a narrow range of 44-52 cm (Fig. 16). viewed with caution.

'Sakagawa, G. T., and A. L. Coan. 1976. State STATE OF THE STOCKS of the skipjack tuna stocks of the Atlantic Ocean Production model analysis is com­ 6ICCAT. 1977. Proceedings of the fourth regular from production model analysis, 1969-1975. meeting of the council (Provisional), November NMFS, Southwest Fisheries Center, La Jolla, monly used in tuna stock assessments 17-22, 1976. Int. Comm. Conserv. Atl. Tunas, Calif., Admin. Rep. No. LJ-76-25, 14 p. (e.g. Joseph, 1970; Skillman, 1975; Madrid, Spain, 121 p.

8 Marine Fisheries Review mates of potential skipjack tuna yields but probably at a reduced catch-per-unit 100 EASTERN ATLANTIC for the Atlantic (Sakagawa and Mur­ of effort. phy, 1976). In 1975, however, fisher­ The skipjack tuna fishery is also con­ j"C men experienced poor fishing for skip­ centrated in the eastern tropical Atlan­

~ 6( jack tuna and the catch reversed its tic. The catch reached a record high of E 2 upward trend, declining sharply by 47 114,700 metric tons in 1974 then sud­ ~ 40 percent. This was the largest decline denly decreased to 59,500 metric tons '3 / recorded for the fishery and seemed to in 1975. This decline apparently was ~ 20 / ./ signal a decrease in stock abundance due more to reduction in availability owing to fishing. rather than reduction in abundance. The °0 10 20 30 40 50 60 70 In 1976, ICCAT (footnote 6) re­ stocks currently appear to be in good STANDARD DAYS AT SEA (xl03l viewed the situation and concluded that condition. However, future catches can

140 the sharp decline in 1975 was due not to be expected to vary markedly, simply TOTAL ATLANTIC because skipjack tuna characteristically 120 a decrease in stock abundance but to poor availability. It predicted that the fluctuate widely in availability; the .! IOC .~~----­ catch would again be low in 1976 fishery is largely dependent on one or ~ 80 (about 40,000 to 50,000 metric tons) two year classes, which causes instabil­ "0 -;:; 60 /y"V owing to fishing effort being diverted to ity in production; and the amount of o / catching yellowfin tuna. fishing effort exerted on skipjack tuna is g40 ,/ Current estimates of MSAY for At­ dependent on availability and abun­ 20ff lantic skipjack tuna range from 89,300 dance of the co-target species, yel­

°0 10 20 30 40 50 60 70 eo 90 to 118,400 metric tons for the eastern lowfin tuna. 3 STANDARD DAYS AT SEA (l( 10 ) Atlantic surface fishery (Sakagawa and Coan, footnote 3). These estimates Figure 21.-Average yield curves and ACKNOWLEDGMENTS observed data, 1964-75, for the At lantic were derived from production models fishery for yellowfin tuna. and are crude because satisfactory pro­ Our sincere thanks to William H. cedures to adjust the data to assump­ Lenarz and David Kramer of the tions of the model have not yet been Southwest Fisheries Center for reading A minimum size regulation is cur­ developed for Atlantic skipjack tuna. the manuscript and offering construc­ rently in effect for Atlantic yellowfin Nevertheless, the estimates indicate the tive criticisms. The paper has benefited tuna that prohibits the taking of large general magnitude of the sustainable from their keen eye for details and ex­ numbers of fish less than 3.2 kg in size yield from the stock and that the yield tensive knowledge of tuna fisheries. in excess of an incidental catch of 15 can be quite variable because the Any errors or lack of clarity in the percent. The regulation was approved fishery depends heavily on year classes paper, however, are solely the respon­ by ICCAT in 1972 to prevent reduction that are available for only I or 2 years. sibility of the authors. in the long-term yield through heavy Apparent natural mortality (biological exploitation of small fish. By 1974, and emigration), especially for fish LITERATURE CITED most ICCAT countries had taken older than 2 years, consequently, is necessary legal action to implement the high and there is no current need for Champagnat, C. 1968. Les campagnes thonieres de "peehe fraiche" 'a Dakar (1955-1967). regulation. About 11,000 metric tons in instituting size or catch regulations for Doc. Cent. D'Oceanog. Dakar-Thiaroye, IS, 1974 and 4,000 metric tons in 1975 of conservation purposes. 53 p., A-Q tables. undersized yellowfin tuna were caught Cleaver, F. C., and B. M. Shimada. 1950. Japanese skipjack (Katsuwonus pelamis) by the eastern Atlantic surface fishery fishing methods. Commer. Fish. Rev. (ICCAT, footnote 6). The estimated SUMMARY 12(11):1-27. losses to future yields from the removal The tropical tuna fishery is the largest Coan, A. L. 1976. Length and age composition of of these undersized fish are approxi. skipjack tuna from the Atlantic Ocean, 1968­ tuna fishery in the Atlantic Ocean. In 73. Collective Vol. Sci. Pap., Inter. Comm. mately 10,000 and 4,000 metric tons. 1975, it produced approximately Conserv. Atl. Tunas., (SCRS-1975) 5(1): 133­ In other words, if the undersized fish 174,000 metric tons of yellowfin and 141. were not caught in 1974 and 1975, they Fisheries Agency of Japan. 1968. Annual report skipjack tunas. of effort and catch statistics by area on would have provided a total of 21,000 The yellowfin tuna fishery is concen­ Japanese tuna longline fishery, 1966. Fish. metric tons and 8,000 metric tons to the trated in the eastern tropical Atlantic. Agency Jap., Tokyo, 293 p. fishery in later years. Growth of the fishery has been princi­ Fisheries Agency of Japan. 1969. Annual report of effort and catch statistics by area on pally in the surface fishery since 1969, Skipjack Tuna Japanese tuna long line fishery, 1967. Fish. when the longline catch stabilized at Agency Jap., Tokyo, 283 p. The catch of skipjack tuna from the about 30,000 metric tons. The stocks Fisheries Agency of Japan. 1970. Annual report of effort and catch statistics by area on Atlantic Ocean increased markedly dur­ currently appear to be in good condition Japanese tuna JongJine fishery. 1968. Fish. ing 1961-74, approaching crude esti- and capable of yielding larger catches Agency Jap., Tokyo, 283 p.

December 1977 9 Fisheries Agency of Japan. 1971. Annual report drid, 74 p. Sakagawa, G. T., and W. H. Lenarz. 1972. of effort and catch statistics by area on ____. 1976b. Proceedings of the fourth American participation in tuna fishery of east­ Japanese tuna longline fishery, 1969. Fish. regular meeting ofthe Commission, November ern tropical Atlantic. Mar. Fish. Rev. 34(11­ Agency Jap., Tokyo, 299 p. 19-25, 1975. Int. Comm. Conserv. Atl. Tunas, 12):55-65. Fisheries Agency of Japan. 1972. Annual report Madrid, 114 p. Sakagawa, G. T., and T. C. Murphy. 1976. of effort and catch statistics by area on Joseph, J. 1970. Management of tropical tunas in Status of the skipjack tuna stocks of the Atlan­ Japanese tuna longline fishery, 1970. Fish. the eastern Pacific Ocean. Trans. Am. Fish. tic Ocean. Collective Vol. Sci. Pap., Inter. Agency Jap., Tokyo, 326 p. Soc. 99:629-648. Comm. Conserv. Atl. Tunas, Madrid, Fisheries Agency of Japan. 1973. Annual report Joseph, J., and T. P. Calkins. 1969. Population (SCRS-1975) 5(1):149-159. of effort and catch statistics by area on dynamics of the skipjack tuna (Katsllwonus Shapiro, S. 1950. The Japanese long-line fishery Japanese tuna longline fishery, 1971. Fish. pelamis) ofthe eastern Pacific Ocean. [In Engl. for tunas. Commer. Fish. Rev. 12(4):1-26. Agency Jap., Tokyo, 319 p. and Span.] Inter-Am. Trop. Tuna Comm. Shiohama, T., M. Myojin, and H. Sakamoto. Fisheries Agency of Japan. 1974. Annual report Bull. 13:273. 1965. The catch statistics data for the Japanese of effort and catch statistics by area on Juhl, R. 1971. Status and potential of the fishery tuna long-line fishery in the Atlantic Ocean and Japanese tuna longline fishery, 1972. Fish. in the Caribbean. Proc. GulfCaribb. Fish.lnst. some simple considerations on it. [In Jap., Agency Jap., Tokyo, 279 p. 23rd Annu. Sess., p. 175-183. Engl. summ.] Rep. Nankai Reg. Fish. Res. Fisheries Agency of Japan. 1975. Annual report Lenarz, W. H., and J. R. Zweifel. In press. A Lab. 21, 131 p. of effort and catch statistics by area on theoretical examination of some aspects of the Skillman, R. A. 1975. An assessment ofthe south Japanese tuna longline fishery, 1973. Fish. interaction between longline and surface Pacific albacore, Thunllus a/a/unga, fishery, Agency Jap., Tokyo, 265 p. fisheries for tunas. Fish. Bull., U.S. 76. 1953-72. Mar. Fish. Rev. 37(3):9-17. Honma, M., and K. Hisada. 1971. Structure of Lenarz, W. H., W. W. Fox, Jr., G. T. Sakagawa, Suarez-Caabro, J. A., and P. P. Duarte-Bello. yellowfin tuna population in the Atlantic and B. J. Rothschild. 1974. An examination of 1961. Biologia pesquera del (Kat­ Ocean. [In Jap., Engl. synop.] Bull. Far Seas the yield per recruit basis for a minimum size suwonus pe/amis) y la a1bacora (Thunnus at­ Fish. Res. Lab. (Shimiza) 4:93-124. regulation for Atlantic yellowfin tuna, Thunnus /anticus) en Cuba I. Instituto Cub. Investiga­ ICCAT. 1973a. Data record, Vol. I. Int. Comm. albacares. Fish. Bull, U.S. 72:37-61. cions Tecol., Serie Estud. Trab. Invest. 15, Conserv. Atl. Tunas, Madrid, 271 p. McNeely, R. L. 1961. The purse seine revolution 151 p. ____. 1973b. Data record, Vol. 2. Int. in tuna fishing. Pac. Fisherman, 59(7):27-58. Uchida, R. N. 1976. Reevaluation of fishing ef­ Comm. Conserv. Atl. Tunas, Madrid, 129 p. Ramos, R., and M. Gerardo Guerra. 1976. fort and apparent abundance in the Hawaiian ____. 1974a. Data record, Vol. 3. Int. Operaciones atuneras con barco-cebo en Ven­ fishery for skipjack tuna, Katsuwonus pe/amis, Comm. Conserv. Atl. Tunas, Madrid, 181 p. ezuela durante 1974. Collective Vol. Sci. 1948-70. Fish. Bull., U.S. 74:59-69. ____. 1974 b. Data record, Vol. 4. Int. Pap., Inter. Comm. Conserv. Atl. Tunas, Wagner, D. P. 1974. Results of Ijve bait and pole Comm. Conserv. Atl. Tunas, Madrid, 121 p. Madrid, (SCRS-1975) 5(1):11-14. and line fishing explorations for pelagic fishes ____. 1975a. Data record, Vol. 6. Int. Sakagawa, G. T., A. L. Coan, and E. P. Holzap­ in the Caribbean. Mar. Fish. Rev. 36(9):31-35. Comm. Conserv. Atl. Tunas, Madrid, 172 p. fel. 1976. Length composition of yellowfin, Williams, F. 1972. Consideration of three pro­ ____. 1975b. Data record, Vol. 6. Int. skipjack and bigeye tunas caught in the eastern posed models of the migration of young skip­ Comm. Conserv. Atl. Tunas, Madrid, 359 p. tropical Atlantic by American purse seiners. jack tuna (Katsuwonus pelamis) into the east­ ____. 1976a. Statistical bulletin, Vol. 6, U.S. Dep. Comm., NOAA Tech. Rep. NMFS, ern Pacific Ocean. Fish. Bull., U.S. 70:741­ 1975, Int. Comm. Conserv. Atl. Tunas, Ma- SSRF-702, 22 p. 762.

MFR Paper 1272. From Marine Fisheries Review. Vol. 39, No. 12, December 1977. Copies of this paper, in limited numbers, are available from 0822, User Services Branch, Environmental Science Information Center, NOAA, Rockville, MD 20852. Copies of Marine Fisheries Review are available from the Superin­ tendent of Documents, U.S. Government Printing Office, Washington, DC 20402 for $1.10 each.

10 Marine Fisheries Review