Interaction between vessels fishing for yellowfin tuna (Thunnus albacares) in the northeastern and southeastern Pacific

Guillermo A. Compeán-Jiménez and Michel J. Dreyfus-León
Programa Nacional de Aprovechamiento del Atún y de Protección de Delfines
U.A. de Nuevo León/Instituto Nacional de la Pesca
Av. Espinoza No. 843, Col. Obrera
Ensenada, Baja California, Mexico

ABSTRACT

An analysis of possible interactions between purse seiners searching for tuna in the northeastern and southeastern Pacific is reviewed mainly with information on total catch of juveniles (length <80 cm) and adults (length >80 cm) in different areas of the fishery and also through statistical correlation. It is shown that large catches of juveniles in some areas could have a negative effect in adult catches in other areas, some years later. These results are preliminary and more research on this field should be encouraged.

1. INTRODUCTION

In the eastern Pacific Ocean (EPO) there are a multiplicity of fleets and nations involved in harvesting yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis). Yellowfin is the most important species taken by the surface fishery in the EPO (70%) and skipjack is the second most important species (20%), other species taken include bluefin tuna (Thunnus thynnus), bigeye tuna (T. obesus), albacore tuna (T. alalunga), bonito (Sarda chiliensis) and black skipjack (Euthynnus lineatus).

In the EPO, fishermen detect tuna in several ways depending on the fishing mode employed, that we can summarize in three principal groups: marine mammal-associated schools, free-swimming schools and log schools (associated with floating objects). Furthermore, there are also differences among fishing modes in the size composition of yellowfin and in fishing areas. For several purposes the purse seine fishery is treated as being made up of two fishing modes, marine mammal-associated schools, and free-swimming and log schools combined (Allen and Punsly, 1984).

The marine mammal schools (tuna associated with dolphins) are composed principally of adult tunas (>80 cm, >10.4 kg) with a large size distribution, but different composition in the number and species of associated dolphins. The free-swimming schools are common in coastal waters and composed principally of juvenile tunas (frequently associated with skipjack). Log schools are composed essentially of very young tunas, in association with other species as skipjack and present mainly south of the EPO.

From 1985 to 1992, the indices of abundance for yellowfin tuna in the EPO show increasing trends, and the catches since 1984 have been even greater than expected (IATTC, 1992b). This was due in part to the fishery concentrating on catching larger fish. Wild (1994) estimated that 36% of the increase in the abundance indices is explained by the reallocation of effort toward the older and larger fish in the stock. However, if the fishermen were to revert to catching mostly juvenile fish (<80 cm, <10.4 kg) the abundance and catches of yellowfin might be substantially reduced. Based on recruitment levels, age-specific catchability coefficients for tuna caught without dolphins and average fishing effort observed during 1980-88, Punsly et al. (1994) predicted that yellowfin catches would be reduced by an average of about 25%.

There are a number of possible interactions that may exist between and among the various tuna fleets operating in the eastern Pacific. Kleiber (1994) noted that interaction can occur between fisheries harvesting the same or different size classes or life stages of the target species. Because in the last ten years the dominant type of school in the whole fishery, has been tuna associated with dolphins, thereby we can assume that the stock conditions can be damaged if the predominant type of school is changed with an effect in the tuna size and the juvenile mortality.

2. TRENDS IN RECENT STATISTICS ON YELLOWFIN IN THE EPO

2.1 Mexican Purse Seine Fishery

Until 1985 the extent of the Mexican tuna fishing operations was a direct result of the size and autonomy of the Mexican fleet. The fleet generally concentrated on free schools of yellowfin tuna, which are particularly abundant near the Mexican coastline.

Since its rapid development in the last ten years, the practice of the Mexican tuna fishing fleet has been to operate for the most part in the Exclusive Economic Zone (EEZ) of Mexico and its adjacent areas. Mexican ships seldom operate south of 5ºN or west of 135ºW (Figure 1). A feature that differentiates current operations of the Mexican fleet is that Mexico now concentrates on marine mammals schools; second in importance are yellowfin tuna travelling in free-schools (Figure 2). These fishing practices are mainly determined by bio-geographical considerations since in the oceanic area off Mexico the available tuna is mainly associated with dolphins as was shown by Au (1991). Despite its presence as one of the most prevalent species in the EPO, there are two main reasons for not catching skipjack tuna (Figure 3). One of them is that the concentration of skipjack varies a lot in the north, where the Mexican fleet prefers to operate due to logistics and expenses. The other reason is because there are fewer floating objects (e.g., logs) on the water than in the south of the EPO.

2.2 International Purse Seine Fishery

Other countries operating in the EPO zone, are Venezuela, Vanuatu, Ecuador and the United States of America. Sometimes they fish north and west of the Mexican EEZ, but most of the tuna catch is taken south of 5ºN. Apart from sovereignty reasons, there are probably economic considerations. The international concern for dolphin protection causes principally to those fleets (other than the Mexican fleet) to redirect some effort back from marine mammal schools to free swimming or log schools (IATTC, 1993). In 1993, the IATTC issued 133 "dolphin safe" certificates to purse seine vessels verifying the fact that marine mammals schools had not been encircled during fishing operations (IATTC, 1994).

That number of allowed certificates is higher in comparison to previous years. For instance, 24 in 1990, 67 in 1991 and 88 in 1992 were issued (IATTC, 1992a; 1992b; 1993; 1994). Due to this redirection of effort, the number of dolphins sets made in the southeastern Pacific, on marine mammal schools decreased (Table 1). The estimated total effort, on marine mammal schools was reduced by 33% in 1993 as compared to 1992 (IATTC, 1993).

Figure 1. Yellowfin tuna catch in the EPO by the Mexican tuna fleet in 1994. (Data from Programa Nacional de Aprovechamiento del Atún y de Protección de Delfines (PNAAPD).)

Table 1. Number and percentage of three types of purse seine sets separated by Mexican and non-Mexican fleet. MM=marine mammal-associated school, FS=free school, LS=log set. (Data from PNAAPD and IATTC.)

		Mexican		Non-Mexican		Total
		No.	%	No.	%
1992	MM	5,932	61.1	3,777	38.9	9,709
	FS	1,710	28.1	4.389	71.9	6.099
	LS	183	9.3	1.779	90.7	1.962
1993	MM	3.541	53.2	3,119	46.8	6.660
	FS	3,376	39.5	5,161	60.5	8,537
	LS	281	15.8	1.496	842	1.777
1994	MM	4.600	76.8	1,384	23.2	5.948
	FS	2,116	36.6	3,670	63.4	5.786
	LS	379	27.7	1,371	78.3	1.750

Figure 2. Mexican tuna fleet set type 1992-94, percentage and number of sets. (Data from PNAAPD and IATTC observer programs.)

2.3 Size Composition of the Catch

Length-frequency samples of yellowfin tuna are collected by IATTC personnel at ports of landing. Histograms showing the estimated catches by gear, year and area appear in the Annual Reports (IATTC, 1987; 1988; 1989, 1991; 1992a; 1992b; 1993). The length frequency samples are stratified by market measurement areas. The areas used by IATTC appear in Figure 4.

The length composition of yellowfin tuna caught by the purse seine fishery is similar in different years, except 1988 (Figure 5) but there are some important differences among the areas (Figure 6 a to g). For instance, Areas 4, 5 and 6, the zone of operation of the international fleet, show in most years higher catches of juvenile fish (less than 80 cm) compared to adult fish (over 80 cm) than others areas and the percent juvenile catch combined from these same areas goes from 59% to 79%. For some years in Areas 1,5,6 and 8 more than 50% of the catch, was less than 80 cm in length. In Areas 1 and 8 these juvenile catches correspond principally to non marine mammal schools. Sets by the Mexican fleet are done mainly on free swimming schools seldom associated with skipjack and in the other areas, where the international fleet operates, there are principally catches of juveniles frequently associated to skipjack or in log schools, in both cases there are very young tunas.

Figure 3. Skipjack tuna catch by country, 1992-94. (Data from IATTC.)

Figure 4. Area used by the IATTC for sampling lengths of tuna in the EPO.

Figure 5. Yellowfin tuna catch by size, 1986-92. (Data from IATTC annual reports.)

Figure 6. Yellowfin catch by size and area, 1986-89. (Data from IATTC annual reports.)

3. INTERACTION BETWEEN THE NORTHEASTERN AND SOUTHEASTERN PACIFIC FISHERIES

The effect of fisheries in one EEZ on catch rates in another EEZ is particularly important for countries with domestic fleet operating mainly in their EEZ and adjacent areas. The recent relocation of fishing effort toward no marine mammal schools by the international fleet caused concern among the Mexican shipowners that their catch rates would be affected.

3.1. Inference on the Interaction

As previously mentioned in the section 1 and 2.3 the difference in dominant fishing modes has an effect on the size composition of the catch. As a way to detect the effect of increasing catches in non marine mammal schools, a multiple regression analysis between average weight (kg) of yellowfin in EPO (Tomlinson et al. 1992 and IATTC, 1993) and catches among different type sets (IATTC, 1994), was done with a time series from 1961 to 1993. The results are shown in Table 2 and in the following equation:

W = 12.9726 + 0.0000529 (D) - 0.0001444 (L) - 0.0000439 (F)

where:

W = Average weight of yellowfin tuna in EPO
D = Catch of yellowfin tuna in marine mammal sets
L = Catch of yellowfin tuna in log sets
F = Catch of yellowfin tuna in free school sets

Table 2. Analysis of variance of the multiple regression between the mean weight and the catch of yellowfin tuna in three different types of sets.

	DF	Sum of squares	Mean square
REGRESSION	3	187.62459	62.54153
RESIDUAL	29	61.25556	2.11226
	F	29.60881
	Signif F	0.00001
	Rsquare	0.75388

3.2 Correlation Between Juvenile and Adult Catches by Areas

A correlation between juvenile and adult catches was made in the IATTC areas (Figure 4) with two years lag between them, to look for a possible relation due to growth, if young tunas in some areas would migrate and found as adults elsewhere. A significant negative correlation index (-0.9531) was obtained only between juvenile catches in area 5 and the adult catches two years later in area 8 (Table 3). This result should be taken with caution due to the lack of enough annual data.

Table 3. Correlation between juvenile and adult yellowfin catches in IATTC areas with a 2-yr lag between them. A# = area, J = juvenile tuna, A = adult tuna, * = <0.01 significance.

	A1A	A2A	A4A	A5A	A6A	A7A	A8A	A13A
A1J	0.0744	0.7243	-0,3096	0.3495	0.6987	0.8225	-0.1874	-0.6051
A2J	-0.2392	-0.5100	0,6826	0.4735	-0.0279	0.2773	-0.9320	0.1411
A4J	-0.1299	-0.6713	0.5524	0.5789	-0.1300	0.5122	-0.5224	0.6031
A5J	-0.3365	-0.0568	0.2273	0.1740	0.2696	0.1042	-0.9531 *	-0.4375
A6J	-0.0078	-0.4244	0.3438	0.3692	-0.3241	-0.1507	-0.9026	-0.0090
A7J	-0.0080	-0.7119	0.8383	0.6215	-0.2942	0.3947	-0.5982	0.6117
A8J	-0.5151	-0.3808	0.7934	0.2123	0.4190	0.6883	-0.3290	0.3216
A13J	0.2645	-0.4124	-0.1026	-0.2521	-0.8005	-0.9076	0.2510	0.3662

4. DISCUSSION

As was mentioned by Suzuki (1994) it is obvious that the interactions among the fisheries on yellowfin tuna are more complicated than previously assumed. An important aspect is the environmental changes which can affect the vulnerability of yellowfin tuna (Wild, 1994). In this particular case, the knowledge of yellowfin tuna migrations in EPO is inadequate especially for adult tunas (Deriso et al., 1991). Nevertheless, our results should make us aware that interactions might not always be found at a global scale but can nevertheless exist at a smaller scale. For those reasons a predominant juvenile size on the harvest of yellowfin tuna could affect the yield in other areas, although this has to be corroborated with further studies and a longer series of data.

5. ACKNOWLEDGEMENTS

We would like to thank M. Hall from the IATTC tuna-dolphin program for providing us some of their statistics for this report, and the editors for providing counsel during preparation of the manuscript.

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