Ashley J. Mullen
Inter-American Tropical Tuna Commission
c/o Scripps Institution of Oceanography
8604 La Jolla Shores Drive
La Jolla, California 92037 USA
Noel Barüt
Bureau of Fisheries and Aquatic Resources
Arcadia Building 860, Quezon Avenue
Quezon City 1100 Philippines
Bachtiar Gafa
Research Institute for Marine Fisheries
Jalan Krapu No. 12
Sunda Kelapa, Jakarta 14430 Indonesia
ABSTRACT
The tuna fisheries in the Philippines and Indonesia represent a wide range of gear types including handliners, gillnets, baitboats and purse seiners. The present study was undertaken to determine if the data available from these fisheries were adequate to detect interactions between fisheries, and especially between the fisheries of Indonesia and Philippines. The results indicated that the data available from the tuna fisheries of Indonesia were inadequate, and that the data from the Philippines, while better, were not up-to-date. The conclusion was that the tuna fisheries systems in the two areas were so complex that catch and effort data alone would probably not be sufficient to model the system properly.
1. INTRODUCTION
A tour of the Philippines and Indonesia was made in November and December of 1993 to determine what data are available concerning tuna fisheries in the region. The purpose was to determine whether the data are sufficient to model the interactions between fisheries zones, and to make recommendations for improvement if the data are not sufficient. The primary interest was in the interactions between the fisheries of the two countries, so the northern part of Indonesia and the southern part of the Philippines were visited. Figure 1 gives shows the region concerned.
Tunas are taken in this region using a complex mixture of different methods, including handlines, gillnets, baitboats and purse seiners. Fish aggregation devices (FADs) are used extensively throughout the region by all types of vessels. The areas of operation overlap to a considerable extent, but are rarely completely coincident. The Philippines has the more complex mixture of gear types, but pole-and-line baitboats, which are common in Indonesia, are rare or non-existent in the Philippines. In their place are many small purse seiners and ring netters; some such vessels also fish Indonesian waters.
Figure 1. Map of the region concerned.
The small purse seiners and ring netters from the Philippines operating within the Exclusive Economic Zone (EEZ) present something of a quandary. They operate largely by agreement with the Indonesian government. Many of them land and process their catch in Indonesia and might be considered part of the Indonesian fishery. It was clear from our visit, however, that many Indonesians involved in the fishing industry are particularly concerned about what they perceive as competition from the Philippines. It seemed reasonable to consider these interactions, and other gear-type effects within a single EEZ, as intra-national rather than international interactions.
We concentrated on those interactions created when one fishery pre-empts another, that is, when one fishery reduces the exploitable fish population of another. This causes a reduction in catches of the second fishery, whatever the rate of exploitation in the second fishery. Other interactions also exist; some are direct, such as the independent use of FADs constructed and positioned by other boats. Others are less direct, such as the construction of canneries with a capacity greater than that of the local fleet. It was suggested to us that this might lead to a change in government policy. We have not attempted to evaluate this supposition, as that would be beyond the scope of this study. If such a policy change were envisaged, we would hope those concerned would consider the effects on the stocks of tuna. We therefore discuss whether the implications of such a hypothetical policy change might be anticipated.
Interactions of this type are determined by the abundance of each fish population and the rate of movement in both directions between them. Interactions are rarely symmetrical; that would require similarly sized populations and a balanced flow between them. There may also be some temporal distortion in the effects between fisheries. We can reasonably consider different gear types operating within the same region to be different fisheries if they have different age-specific selection profiles. In such cases it is likely that interaction will be asymmetric and temporally distorted. The gear that selects for older fish is likely to be more affected by the other gear than vice versa, and this effect will be delayed by the growth period of the fish.
Estimates of abundance could come either from an analysis of the estimated age structure of the catch, or from biomass models. Age-based analysis was pioneered independently by Murphy (1965), who named it cohort analysis, and Gulland (1965), who coined the term virtual population analysis (VPA). These methods require a sampling programme to determine the age structure within the catch. Generally, length frequencies are determined by measurement, and these are converted to age frequencies by means of a length-at-age table. Biomass models need much less detail concerning the catch, requiring only the total weight, but they additionally require estimates of effort.
Tagging experiments have been conducted in the region, but returns were still being made during this study and the analysis was not complete. Preliminary data from tagging studies show that skipjack move inside the Indonesian archipelago from the EEZ of the Philippines (Hampton, 1993). Given the dispersive nature of the fish's behaviour, it is likely that movements occur in the opposite direction. The interactions between fisheries of the two zones are determined by the rate of exchange in each direction, and by what that exchange represents as a fraction of the quantity of fish in each zone.
Neither yellowfin nor skipjack appears to perform directed migration; there is no clear consistent seasonal pattern to their movements. Their movement is characterized as dispersive, and the rate of dispersal is not consistent either. Mullen (1989) discussed these issues for yellowfin and argued that the variable rate of dispersal is a behavioural adaption that allows individuals to spend more time in better areas.
2. TUNA FISHERIES IN THE PHILIPPINES
The fisheries, and the data collection system created by the Philippines Bureau of Fisheries and Aquatic Resources (BFAR) in collaboration with the Indo-Pacific Tuna Management Programme, are detailed and extensive. The data are not complete, however, in the temporal sense. In December 1993 there was a two-year backlog of data, i.e., data for two years had not been entered into the system and provided to IPTP.
The data collection system has been described by Morón (1992). Employees of the BFAR routinely gather catch and effort data, and also measure samples of particular species taken from individual boats. All of the major ports and landing sites are sampled, but the large number of artisanal boats operating from beaches scattered all around the islands suggest that a significant proportion of the catch may go unmonitored.
The smallest vessels consist of hand-powered canoes which may be used to deploy bag nets, gillnets or handlines. Larger outrigger canoes are fitted with engines which substantially increase their range. Larger vessels of approximately 15-25 tons are employed as purse seiners and ring netters. The largest vessels--the superseiners--exceed 1,000 tons (Aprieto, 1995).
The catch of bag nets and gillnets do not contain a high proportion of tunas, and will not be considered further in this paper. Superseiners will also not be considered further, as they tend to operate in distant waters. Hampton and Bailey (1993) discuss operations by superseiners.
There is some confusion as to nomenclature of purse seiners and ring netters. BFAR classifies any vessel with a power block as a purse seiner. Mr. Amadeo of Amadeo Fishing Co. explained, however, that for them the difference lies entirely in the design of the net. A ring net requires 30-60 men to haul, while a boat equipped with net and a power block would carry 25 men.
The difference between small purse seiners and ring-netters is perhaps moot. Both take fish swimming near the surface. Catches of skipjack and yellowfin are taken in similar proportions by both types of net, and the sizes of both species do not vary greatly between the two net types (see Morón, 1992). The handliners catch fish swimming deeper. Their catch is predominantly of large yellowfin (Figure 2) and bigeye tuna, while that of the surface fishery is of small yellowfin and skipjack (Figure 3). The fact that the largest yellowfin are not caught on the surface, but are taken at greater depths, is not unusual. For example, Nakano and Bayliff (1992) showed that, longlines, which fish deeper water than purse seiners, catch larger yellowfin than the purse seiners in the eastern Pacific.
Figure 2. Relative frequency distribution for yellowfin caught at Lion Beach, General Santos, in 1991.
The preponderance of very small fish in the catches of purse seiners and ring netters shown in Figures 3 and 4 are, however, much greater than those taken by the superseiners in the eastern Pacific. Superseiners operating in the western Pacific, east of the Philippines, also take much larger fish than the surface fishery of the Philippines (Hampton and Bailey, 1993). The frequency of small fish sampled from the Philippines surface fishery suggests fishing mortality might be very high. If so, the much larger yellowfin taken by handliners in the same area must be immigrants arriving as mature adults, rather than survivors of the surface fishery.
Figure 3. Relative frequency distribution for skipjack caught by purse seiners at General Santos in 1991.
There is, however, an alternative explanation. Small "light boats" are used to move fish from one FAD to another. They concentrate fish under a single FAD for a purse seiner or ring netter to make its set close to dawn. It is possible that larger fish are less attracted to the lights and are, thus, less susceptible to this form of fishing. The intermediate-sized fish may be able to evade the nets of the surface fishery. Purse seiners formerly working in the eastern Pacific were unable to fish successfully in the western Pacific until they had developed deeper, faster sinking nets that reached the thermocline, which is deeper in this region. Ring nets are approximately 300 m long and 30-60 m deep. We were told that a typical set takes approximately an hour. Larger fish swim faster and would be able to evade capture more easily.
It should be noted that Aprieto (1995) interprets the size intervals missing from the catch data as evidence for a life-cycle migration. This interpretation holds that yellowfin migrate out of the waters of the Philippines and return as much larger adults.
The data from 1980 presented by Morón (1992) show no consistent decline in catch rates except for Santa Cruz, and the size frequency of catches of yellowfin by purse seiners also appear stable. Again, this can be interpreted in two ways. The population exploited by purse seiners may already have been heavily overfished in 1980, and effort did not decline substantially, so the population has not recovered. Alternatively, the low catch rates of the fish of intermediate size by the surface fishery may be due to the low vulnerability of those fish.
Figure 4. Relative frequency distribution for yellowfin caught by purse seiners at General Santos in 1991.
3. INDONESIAN TUNA FISHERIES
There are many strong contrasts between the fisheries of Indonesia and those of the Philippines. There is no database for Indonesia that is comparable with that of the Philippines. Data are available, but they are suspect for reasons discussed below. We were able to obtain potentially-useful data from commercial fishing companies. In this section we shall concentrate on the pole-and-line, or baitboat, fishery for skipjack.
To the casual observer, it seems there are fewer small landing sites in Indonesia compared to the Philippines, where fishing appears to be conducted from every conceivable point along the coast. In Indonesia, fish are seen as a resource which is not yet fully exploited, and the fishing industry is actively promoted by government as a growth industry. Except for vessels owned in the Philippines and operating within Indonesia's EEZ (but outside the archipelago), there are no purse seiners or ring netters. Nets are used for collecting bait for the pole-and-line fishery that operates here primarily for skipjack. As in the Philippines, yellowfin and bigeye are caught on handlines. In addition, a relatively small amount of these fish are caught on longlines and landed in Bitung. Catches of all these species are primarily for export. There is also some jigging for reef fish, as well as trolling for the local markets. Here, except for the zone where purse seiners and ring netters from the Philippines are permitted, the surface fishery is dominated by pole-and-line vessels.
An important part of the Indonesian government’s approach for stimulating fisheries is the Nucleus Estate for Smallholders, or NES. The nucleus is a state-owned company and the smallholders consist of the owners of individual boats, as well as some who own several boats. The nucleus is in a position to direct activities of all the smallholders. It not only supplies bait and buys the catch of the smallholders, but also constructs FADs and places them in appropriate locations. These locations are based on several criteria--the availability of bait nearby, topography, and seasonal weather and oceanographic conditions. The Southern Monsoon makes fishing impossible from south-facing coasts, and catch rates are highest when operations can be resumed.
Social factors were also criteria in siting of at least one port. Mr. Rossu Hutubarat, director of P.T. Usaha Mina, explained to us in Labuha that the small local population had been part of the reason for expanding facilities there. Previously, Ternate had been an important fishing centre, and most of the boats operating out of Labuha are still based there. It was felt that much of the catch in Labuha was not landed to P.T. Usaha Mina, rather its was distributed directly to the local population.
The pivotal position of the nucleus enabled us to assess the validity of the existing data. It seemed that wherever there was the opportunity to compare data, there were serious discrepancies. For instance, the positions given as locations where catches had been made by baitboats seemed anomalous. We interviewed Mr. Abidin, the fishing operations manager for P.T. Usaha Mina, who told us that no boats associated with his company had ever fished where the catch was recorded as having occurred. These data were collected by workers in processing plant of P.T. Usaha Mina, and they were paid by RIMF for this extra work. In other cases, the total catch was not in concordance with the catch by size and species.
Itano (1993) describes the operations of the pole-and-line skipjack fishery. He also reports that the literature concludes that eastern Indonesia, as a whole, is not fully exploited, although some localities may be. The fishery seems to be limited by the availability of bait, rather than by the availability of tunas. Skipjack are the prime target of this fishery; the low fat content of skipjack in this region make them particularly suitable for the katsuobushi (dried fish) market, and they are exported to Japan. Some of the handline-caught yellowfin and bigeye is exported to Japan for sashimi. A preparation and packing plant has been constructed in Labuha to prepare sashimi ready for supermarkets. It is a joint venture with a Japanese company that had just started trial operations when we were there in November 1993.
The pivotal position of a single company in the pole-and-line fishery, P.T. Usaha Mina, compensates to some extent for the lack of reliable survey data. Their records were made freely available to us, and we were able to have extensive discussions with the officers of the company. The records kept by the company are of the form shown in Table 1. These data are likely to be accurate because they have been gathered for their commercial importance by those who intend to use them.
Table 1. Catch and effort of Sorong baitboat fishery, 1990. The table is for one class of baitboat, nominally 30 tons displacement. Columns are: M, month; Gear, 30 ton baitboats; OPD, operational days effort; bait consumed in “buckets”; kg of yellowfin, and skipjack by weight class (kg) of individuals, and the total catch of tunas.
|
MONTH |
EFFORT |
BAIT |
YF<1.5 |
YF<2.5 |
YF<10 |
YF>10 |
SJ<1.0 |
SJ<2.5 |
SJ>2.5 |
TOTAL |
|
1 |
282 |
16993 |
0 |
4715 |
48392 |
3663 |
80908 |
81669 |
173267 |
392614 |
|
2 |
343 |
20374 |
|
2248 |
37363 |
1792 |
74154 |
56764 |
399310 |
578070 |
|
3 |
408 |
23108 |
26486 |
6221 |
14221 |
26131 |
39445 |
67983 |
502674 |
706930 |
|
4 |
428 |
22383 |
15715 |
2676 |
20545 |
5236 |
76113 |
81701 |
410597 |
612583 |
|
5 |
494 |
30445 |
18119 |
15529 |
43289 |
827 |
206078 |
134998 |
390980 |
809820 |
|
6 |
422 |
24377 |
5538 |
6395 |
39077 |
151 |
46724 |
76531 |
289483 |
463099 |
|
7 |
528 |
26850 |
12370 |
14634 |
41978 |
1411 |
47342 |
68539 |
117446 |
303720 |
|
8 |
600 |
31566 |
|
6831 |
89184 |
8716 |
21690 |
30019 |
357468 |
513908 |
|
9 |
560 |
34470 |
5267 |
15187 |
138753 |
14465 |
44433 |
33689 |
437953 |
609747 |
|
10 |
652 |
39241 |
3080 |
3636 |
168457 |
6988 |
130292 |
102555 |
440187 |
855195 |
|
11 |
587 |
35674 |
7833 |
4960 |
117927 |
5295 |
58632 |
78764 |
428866 |
714026 |
|
12 |
514 |
34712 |
9850 |
28356 |
110910 |
4486 |
57328 |
117501 |
459906 |
788537 |
Bitung, on the northeast coast of Sulawesi, is the main port for boats from the Philippines that land in Indonesia. There are several recently constructed canneries. The data that were made available to us by some of the canneries showed that the size distribution of the catch was similar to that found for catches made by similar boats in waters of the Philippines. Catches of the larger baitboats (up to 50 tons) in this region seem to be stable at present. It was reported to us that the smaller boats face difficulties catching bait on the north side of Sulawesi, which might be due to development that has occurred since the construction of a new road. Itano (1993) reports that pole-and-line fishermen of this region view as a resource the FADs placed by operators of purse seiners and ring netters. Mr. Rodriquez of RD Fishing in the Philippines told us however that their smaller purse seiners (they also operate superseiners) stay at least 50 miles from the Indonesian coast. This distance is maintained, he said, to minimise the interactions with Indonesian boats. Their licence entitles them to fish up to 12 nm from the coast.
4. DISCUSSION
It was stated that the data available for Indonesia were inadequate, and that those for the Philippines were much better although these were not up-to-date. In this section we discuss possible improvements that might be made to the data collection system, and the possible limitations of even a very good data set given the complexity of the system. Finally, we make suggestions given the information that are at our disposal.
Figure 5. Proportion of skipjack in the largest size category that were caught in Sorong.
Figure 6. Proportion of skipjack in the largest size category that were caught in Fak Fak.
Figure 7. Map showing relative position of Sorong and Fak Fak in the northwest of Irian Jaya, and the locations of FADs.
The data from the Philippines should be transcribed from their raw form to computer files as quickly as practicable. This is not only so that they are available for analysis, but also so that apparent anomalies might be questioned while personnel are available and memories are fresh.
Currently, the boat captains are interviewed after trips and asked where they fished. If they operated outside the Philippines EEZ, this is a single category. Many of those boats will have been operating in the EEZ of Indonesia. Some land their catch in Indonesia, in which case the information should be gathered there. Others land in the Philippines, and it would be useful if all the data gathered for Philippine waters were also gathered from those boats that have worked in Indonesian waters. There may be problems in implementing this recommendation--the data are of more interest to the Indonesian government than to that of the Philippines, and some boats may have been fishing illegally. Gathering data concerning illegal fishing is often a problem, and one that might be overcome by some protection from self-incrimination. Such protection is believed to be effective, but we can offer no useful advice on this point.
It is hard to assess the impact of some of the smallest boats that use a variety of gear types and either consume their catch directly, or market it in ways that are difficult to monitor. An investigation would require a combination of careful anthropology, detailed sampling of the catch, and careful analysis of the statistics.
Other than the points noted, the data collection scheme in the Philippines provides a good model for Indonesia. That is not to say that the Philippines scheme can be copied easily. In a region where the samplers are necessarily dispersed, it must be hard to impart the attitude that the data are important, and much will depend upon the individuals that set up the system. Monitoring and questioning of unusual data might help, as would some feedback such as graphed time-series, etc. This is a difficult management problem, and we claim no special skill in that subject.
It should be noted that, even with complete and detailed information, this region would present a very difficult task for investigating interactions. The region is extremely complex in almost every aspect. The region is situated in a monsoon region. This, coupled with the very variable nature of the topography causes the oceanography of the region to be complex, both spatially and temporally. The weather associated with the monsoon imposes constraints on fishing operations, so strata without catches do not necessarily indicate few fish. Added to the physical and environmental constraints, are social issues. Market prices change and cause different size classes to be preferred by fishermen. Fleets in Indonesia are directed away from centres of population, while boats from the Philippines stay further than legally required from the coast of Indonesia. These problems are a general part of fisheries science, but we cannot think of another region that has them to the same degree.
A full assessment of interactions requires a complete spatio-temporal model of the region. Until relatively recently, fisheries models were spatially aggregated, which implicitly assumes a homogeneous distribution of both fish and fishing effort. If this is invalid then very poor estimates can result (Mullen, 1994). Spatio-temporal models exist for other regions, e.g., Mullen (1996) and Mullen et al. (1996). The spatio-temporal models described are for a much simpler region that is relatively well sampled, and are far from complete.
Boggs (1994) considered a more analogous problem, the fisheries of Hawaii. He considered a single pelagic population from which the fish of twelve localities were recruited, still an over-simplification for the fisheries considered here. Each local population in Boggs' model was subject to natural and fishing mortality upon immigration. The variation in fishing mortality caused local catch and catch per unit effort (CPUE) to vary through time differently. The pelagic population was considered constant at first, so every locality experienced constant immigration. In this case, he was able to establish relationships between the local CPUEs and catches. On relaxing the assumption of constant immigration by allowing it to decline by 40%, this relationship was weakened considerably. Boggs' conclusion from his simulation experiments was that catch and effort relationships might vary seasonally and annually. He goes on to say that CPUE versus effort should at least be useful in detecting the impact of local fishing on local CPUE, but that detecting the impact might require quantifying the effects of other factors.
Estimation of parameters is often a problem in fitting fisheries models to data. Models are frequently over-parameterised. Ludwig and Walters (1985, 1989) showed by simulation that, when used for management, simpler biomass models could sometimes out-perform more complex age-structured models that were known to be more realistic. This occurred when the data were uninformative and the parameters were obtained with little precision. The spatio-temporal model we envisage would make the age-structured model seem simple because it would also involve age-dependent movement parameters that might be density dependent and/or vary through time. Clearly, such a model cannot be constructed now, and it might never be possible without exogenous determination of the movement parameters by, say, tagging experiments.
The work of Ludwig and Walters (1985, 1989) should be seen positively however. Simple models can give useful management advice. One of the first methods of analysis for fisheries was yield per recruit, and we propose to discuss the problem of interactions in this area using that idea.
Walters (1986) argues that strong signals imparted to natural systems can aid the analysis of that system. Such a signal may be imparted to the system soon, and this might be sufficient to ascertain the degree of interaction between the purse seiners and ring netters on one hand, and baitboats on the other. New canneries are planned for Bitung and Sorong, so the capacity of canneries is soon likely to exceed the catches of baitboats. There is speculation that, as a result, purse seiners and ring netters will be permitted to operate within the archipelago of Indonesia, where they are currently excluded. The experience of the Philippines suggests that the handliners may not be unduly affected by such a change. Indeed, yellowfin tuna may be grossly under-exploited in this region, although this may be part of the reason for the good catches in the Philippines. It is the baitboats that are likely to be affected, first and most. If so, then data of the sort shown in Table 1 would be sufficient to detect any substantial effect, but the magnitude of the effect cannot be predicted.
Clearly, the small fish taken by purse seiners and ring netters would lead to a low yield per recruit if most of the catch came from those boats. The important question is how large the overall populations of skipjack and yellowfin are within the archipelago of Indonesia. The current pole-and-line fishery gives little information because it concentrates on skipjack and is limited by bait, and the evidence of Figures 5 and 6 suggest the skipjack population is not uniformly distributed.
Unfortunately, changes in the catch rate of skipjack induced by enhanced mortality of young fish may have non-linear effect on the industry. The pole-and-line fishery is not economic at the current world price for skipjack, but it is profitable because the catch commands a higher price for katsuobushi (Hutubarat, P.T. Usaha Mina, pers. comm.).
In conclusion, we can say that the fisheries data alone are insufficient to assess the degree of interaction that exist between the two EEZs considered, or between surface netting and handlining for tunas. The system for gathering data could be improved, but there is no guarantee that even perfect catch and effort data would provide sufficient information for modelling such a complex system. There is no clear indication of a strong effect by purse seiners and ring netters on pole-and-line baitboats in the one zone where they now overlap. The low yield per recruit of the net fishery is a cause for serious concern, but the effects cannot be predicted until an estimate of the exploitable population is determined.
5. ACKNOWLEDGMENTS
We would like to thank all of the people from many fishing companies who freely gave us both their time and data. Mr. Zainal Abidin of P.T. Usaha Mina was especially generous with his time. The senior author (AJM) would like to thank the Director of the IATTC, Dr. James Joseph, for allowing him to take a leave of absence to perform this work. This study was conducted for the Indo-Pacific Tuna Development and Management Programme under the auspices of the United Nations Food and Agriculture Organization. Finally, we appreciate the helpful comments of Purwito Martosubroto and the editors.
6. REFERENCES
Aprieto, V.L. 1995. Philippine Tuna Fisheries: Yellowfin and Skipjack. University of the Philippines Press, Diliman, Quezon City, Philippines. 251 p.
Boggs, C.H. 1994. Methods for analysing interactions of limited range fisheries: Hawaii's pelagic fisheries. In: Shomura, R.S., J. Majkowski and S. Langi (eds.). Interactions of Pacific tuna fisheries. Proceedings of the First FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 3-11 December 1991, Noumea, New Caledonia. Vol. 1: Summary report and papers on interaction. FAO Fish. Tech. Pap. (336/1): 74-91.
Gulland, J.A. 1965. Estimation of mortality rates. Annex to Arctic Fisheries Group report. (ICES, C.M. 1965. Doc. No. 3)
Hampton, J. 1993. Exploitation rates in the Philippines domestic tuna fishery: estimates from the Philippine Tuna Research Project tagging study (PTRP DFR). South Pacific Commission, New Caledonia. 15 p.
Hampton, J., and K. Bailey. 1993. Fishing for tunas associated with floating objects: a review of the western Pacific fishery. Tuna and Billfish Assessment Programme, Technical Report No. 31. South Pacific Commission, Noumea, New Caledonia. 48 p.
Itano, D.G. 1993. The development of the Indonesian pole-and-line fishery in relation to the efficient utilization of live baitfish resources. Phase 1: Field survey of tuna baitfish capture and handling techniques in eastern Indonesia. Research Institute for Marine Fisheries, Jakarta, Indonesia. 33 p.
Ludwig, D., and C.J. Walters. 1985. Are age-structured models appropriate for catch-effort data? Can. J. Fish Aquat. Sci. 42: 1066-1072
Ludwig, D., and C.J. Walters. 1989. A robust method for estimation from catch-effort data. Can. J. Fish Aquat. Sci. 46: 137-144
Morón, J. 1992. The tuna sampling programme in the Philippines. Fifth Southeast Asia Tuna Conference, Indo-Pacific Tuna Development Programme, Colombo, Sri Lanka.
Mullen, A.J. 1989. Aggregation of fish through variable diffusivity. Fish Bull, U.S. 87: 353-362.
Mullen, A.J. 1994. Effects of movement on stock assessment in a restricted-range fishery. Can. J. Fish. Aquat. Sci. 51: 2027-2033
Mullen, A.J. 1996. A method to estimate movement from changes in estimated distributions, and then revise those estimates. In: Shomura, R.S., J. Majkowski and R.F. Harman (eds.). Proceedings of the Second FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 23-31 January 1995, Shimizu, Japan. [This volume]
Mullen, A.J., A.A. Anganuzzi, R.D. Deriso, R.G. Punsly and G.J. Walker. 1996. Interactions between mode of fishing and category of vessel in catches of tunas in the eastern Pacific tuna fishery. In: Shomura, R.S., J. Majkowski and R.F. Harman (eds.). Proceedings of the Second FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 23-31 January 1995, Shimizu, Japan. [This volume]
Murphy, G.I. 1965. A solution to the catch equation. J. Fish Res. Bd. Can. 22: 191-202.
Nakano, H., and W.H. Bayliff. 1992. A review of the Japanese longline fishery for tunas and billfishes in the eastern Pacific Ocean, 1981-1987. IATTC Bulletin 20(5): 355 p.
Walters, C.J. 1986. Adaptive Management of Renewable Resources. MacMillan, New York. 374 p.
