© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Behaviour of the southern sea lion (Otaria flavescens) and consumption of the catch during purse-seining for jack mackerel (Trachurus symmetricus) off central Chile
Departamento de Oceanografía, Universidad de Concepción PO Box 160-C, Concepcion, Chile
*Correspondence to L.A. Hückstädt; tel: +56 41 204345; fax: +56 41 256571. e-mail: lhuckst{at}udec.cl.
The fishery for jack mackerel (Trachurus symmetricus) off central Chile competes for the resource with southern sea lions (Otaria flavescens), and during purse-seining makes the fish more accessible to the pinnipeds. Interactions with sea lions were recorded during 31 purse-seine sets off central Chile during October 1999. The sea lion behaviour associated with the fishing operations was distinctive. Feeding, movement, predator avoidance, and resting displays were identified. The sea lions approached the purse-seiner as soon as net-setting began. The number of sea lions per set (0–50) was seemingly unaffected by school size of jack mackerel, number of purse-seiners on the fishing ground, whether fishing was by night or by day, the presence of killer whales, or the species being targeted. However, the number of sea lions at a purse-seine differed significantly between fishing grounds. Other effects of fishing operations on O. flavescens included incidental mortality and capture. The amount of fish consumed by the sea lions at a set was as much as 0.4% of the catch. The results of the interaction are documented and discussed in the light of likely interaction with the whole sea lion population, as well as the impact of the interaction on the fishery.
Keywords: behaviour, central Chile, Otaria flavescens, purse-seine fishery, Trachurus symmetricus
Received 18 July 2002; accepted 27 April 2003.
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Introduction |
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The main food pathway in the Chile–Peru Current, particularly in the south where jack mackerel (Trachurus symmetricus) is mainly caught, centres on an intermediate herbivore (Euphausia mucronata). E. mucronata is the dominant prey of wide spectrum predators such as jack mackerel, whiptail hake (Macruronus magellanicus), hake (Merluccius gayi), snoek (Thyrsites atun), and other pelagic predators (Antezana, 2001).
The Chilean industrial fishery for jack mackerel increased from some 600 000 t during 1980 to more than 4 million tonnes during 1995, before decreasing to about 1 million tonnes in 1999 (Sobarzo and Cubillos, 1998; SERNAPESCA, 1999). Such heavy fishing pressure impacts not only the target populations, but also the entire ecosystem of predators and prey of jack mackerel (Antezana, 2001). Among these impacts will undoubtedly be one on the southern sea lion (Otaria flavescens) because jack mackerels are the dominant prey of sea lions off southern and central Chile (George-Nascimento et al., 1985).
The jack mackerel fishery probably affects the predators in several ways (Beverton, 1985), one of which would be on the feeding behaviour of southern sea lions. Although there is some information in the literature about how O. flavescens interacts with small-scale fisheries and aquaculture off Chile (Aguayo and Maturana, 1973; Torres, 1979; Oliva, 1984; Oporto et al., 1991; Sepúlveda, 1998), we are unaware of any literature on operational interactions with an industrial fishery. However, confronted with sudden and intense aggregations of its prey in purse-seine nets, we would expect pinniped feeding behaviour to change. Our hypothesis is therefore that the traditional search and pursuit behaviour of sea lions would change to one of sitting and waiting for the next "free lunch" provided by the purse-seiners.
This study is a first attempt to describe such feeding behaviour as induced by the industrial jack mackerel fishery, a relatively recent interaction off Chile. We also attempt to estimate the impact of sea lion feeding on the total catch and populations of jack mackerel and to speculate on the importance of the fishery in the dynamics of sea lion stocks off Chile.
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Material and methods |
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Sampling was conducted from one commercial purse-seiner of the central Chilean fleet based around Talcahuano-Coronel during October 1999 (Figure 1). The area of operations of the fleet (35°35'S–38°38'S) is a coastal belt up to 50 miles offshore adjacent to a narrow continental shelf (ca. 5–10 miles) and a coastline exposed to the dominant southwesterly winds and the Chile–Peru Current. There are 10 sea lion haul-outs within the area of purse-seine operations. The purse-seiner used as a platform for the observations formed part of a total fishing fleet of 20 vessels operating during the study period that had a hold capacity and total length near the top of the range of the fleet. The length of net it deployed was therefore also one of the largest in the fleet. Observations were made on the main fishing grounds (Nugurne, Achira, La Feria, Punta Lavapié, Morguilla, North Isla Mocha, and South Isla Mocha) during 31 sets.
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At each set, sea lions were counted during four stages of the operation: setting, retrieval, transfer of the catch onboard, and departure from the fishing site. The count was made every 10 min for 1 min during setting and retrieval of the purse-seine and for 10 min after completion of retrieval, covering a radius of observation of about 500 m around the vessel. A 10–30x30 binoculars was used for initial counts and the results checked by video records later. The duration of purse-seine operations averaged 90 min, with a range of 70–130 min. For night sets, observations were made only after deck lights were turned on during purse-seine retrieval. Observations were restricted to animals at the sea surface, around and inside the purse-seine net. Additional information was recorded on location, school size, fish composition, catch volume, and depth. Additional counts of pinnipeds were made during navigation to and from the fishing grounds.
Observations on the behaviour of O. flavescens were focused on single animals, which were followed until they dived, recording all observed displays. Observation was switched to another animal selected at random when the observed animal dived. This methodology is a qualitative approach because it does not take into account the number of animals presenting any specific display at a given time or the time invested in a given display, only a specific display actually occurring. For this work, we adopted Wilson's (1975) definition of display as any behaviour that conveys information, regardless of whether it serves other functions.
The monthly operational consumption of T. symmetricus by O. flavescens per fishing vessel (Em) was estimated according to the equation of Wickens et al. (1992):
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Results |
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Distribution of sea lions during purse-seining
The number of sea lions observed at a purse-seine ranged between 0 and 50 per set, with an average of 21 (Table 1). The number observed per set varied significantly over the study area (Kruskal–Wallis test; p=0.047), as well as among fishing grounds (Table 1). Sea lions were fewest at purse-seine operations in the north (Nugurne) and most abundant on the southernmost fishing ground (South Isla Mocha), but there was no obvious latitudinal trend.
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The number of sea lions observed during a purse-seine set (Figure 2) increased on all fishing grounds with time of the net in the water, with zero or few sea lions at the beginning of the set and maximum numbers towards the end. However, at night, there was generally a time-lag of up to 50 min between the start of a set and the start of the sea lion count; counting from the beginning of the set was not possible because deck lights were off. Numbers of sea lions were not significantly different between sets made by night and by day (p=0.27; Mann–Whitney U-test; Table 2).
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There was no significant relationship between the number of sea lions and jack mackerel school size (linear regression, p=0.36; Figure 3a), number of vessels on the ground (p=0.24; Figure 3b), or distance to the nearest haul-out (p=0.29; Figure 3c).
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During purse-seine operations, killer whales, Orcinus orca, were occasionally attracted into the area (four out of 31 sets), presumably to prey on the sea lions. The assumption of predation is based on observations of sea lion escape behaviour in the presence of killer whales, the behaviour of the approaching killer whales during the operation, and the skipper and crew's testimony that they had observed killer whale predation on O. flavescens during purse-seining. However, the number of sea lions at a purse-seine was not significantly associated with the presence or absence of killer whales (p=0.48; Mann–Whitney U-test).
Occasionally, whiptail hake (M. magellanicus) were the target species of the purse-seine. Sea lions were also attracted to such purse-seines and fed from it; indeed, there was no significant difference in the number of sea lions attending jack mackerel and whiptail hake sets (p=0.29; Mann–Whitney U-test).
Behaviour of sea lions during purse-seining
The behaviour of sea lions during daylight purse-seine operations was recorded by video camera from the fishing vessel. Observations were made through most of the region. Displays were identified (Table 3) and counted every 10 min from video recordings and computed as a function of elapsed time from the beginning of each set to departure from the fishing site. The presence of sea lions throughout the fishing operation allowed for uninterrupted recording of displays. In all, 14 video observations were made during the entire study period.
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The total number of displays showed a bell-shaped pattern, with minima near the beginning and end of the fishing operation and a maximum about 70 min after starting a set. Towards the extremes, some displays were infrequent (e.g. those associated with feeding) owing to the dominance of movement displays associated with the beginning of purse-seining and predator avoidance. The most frequent displays throughout the fishing operations were flat swimming and diving.
In order to discriminate the effect of successful fishing on the behaviour of sea lions, the number of displays was compared between successful and unsuccessful sets (Figure 4a), and among behaviour categories (Figure 4b–d). Predator-avoidance displays were excluded from this analysis because they were performed exclusively when O. orca was present. In successful sets, the number of feeding displays was distributed in a bell-shaped curve, with a maximum 40–90 min after the start of the fishing operation. During unsuccessful operations, feeding displays were delayed towards the end of the set when a purse-seine with few or no fish was almost retrieved. Movement displays also varied between successful and unsuccessful operations. The maximum number of such displays was during unsuccessful operations and coincided with the end of the operation. In contrast, movement displays during successful operations were observed constantly and with little variation throughout the operation.
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Resting displays followed a similar bell-shaped trend when sets were unsuccessful, but there was no clear trend during successful sets.
Impact of sea lion feeding on the purse-seine catch
The October 1999 studies actually encompassed 42 sets (Hm), although observations were made on only 31. Individual mean daily ration was assumed to be 10.8 kg (George-Nascimento et al., 1985), so the operational consumption (Em) calculated from Equation 1 was 42 hauls x 21 sea lions x 10.8 kg, or 9.525 t vessel–1 month–1. The fishing vessel caught 2390 t during the sampling period, so the operational consumption by sea lions was 0.39% of that vessel's catch made then.
Of course, the catch of the sampling vessel cannot be considered as an average for the whole fleet because its hold capacity, total length, and length of net deployed are near the top of the range for the whole fleet. Therefore, any extrapolation to a fleet level should be done assuming an average number of sets for a standard vessel, and hence considering that the number of purse-seine sets is a function of prey encounter rather than a function of the parameters of the fishing vessel. A standard fishing vessel, with a hold capacity of about 500 m3 (Hernández and Sepúlveda, 1998), performs an average of 25 fishing sets per month. During October 1999, 20 fishing vessels were operating and, according to official reports by SERNAPESCA (Chilean National Fisheries Service), 25 000 t of jack mackerel were caught. Therefore, a provisional estimate of the impact of sea lions on the catch of the entire fleet should be based on the total number of sets performed by the fleet (25 setsx20 vessels), the average number of sea lions per set, and the average operational consumption by sea lions. Again using Equation 1, the calculation would yield 500 setsx21 sea lions set–1x10.8 kg sea lion–1 or 113.4 t, approximately 0.45% of the total catch.
There was no other direct impact of sea lions on the fishery, e.g. damage to the net, or influence of the pinnipeds on fish schooling behaviour and catchability.
Impact of fishing on the survival of sea lions
The fishery had two types of impacts on the sea lions: mortality (animals killed during the purse-seine operation) and capture (animals trapped inside the purse-seine when the net contents were pumped aboard and subsequently released alive).
During the study period, two sea lions were killed and 18 captured (identified as juveniles by observers on board). One of the captured animals was seriously injured, with dislocation of the lower jaw and fracture of the left foreflipper, which would have led to its death later.
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Discussion |
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There is clear operational interaction between the Chilean jack mackerel fleet and southern sea lions during commercial fishing. Our results show that sea lions benefited from being able to feed in close association with the fleet as a result of the herding effect of purse-seining. Sea lions approached the vessel soon after fishing operations started and increased in numbers until the haul ended. Moreover, sea lions did not approach the vessel until purse-seine setting was underway, which indicates perhaps that either the acoustic detection device or more likely the noise of the purse-seine being shot was effectively attracting them to the vessel. At a larger scale, sea lions probably have other means (e.g. knowledge/memory, presence of other sea lions) of locating the main fishing grounds.
The differences in sea lion density between fishing grounds may be associated either with distance from haul-out or with a learned knowledge of the productivity of the fishing grounds. Because there is a poor relationship between the presence of sea lions at a fishing operation and distance from haul-out, we suspect the latter to be a more likely explanation. The maximum number of sea lions was recorded on the southernmost fishing ground (South Isla Mocha), which has the highest historical landings. In contrast, Nugurne (in the north) was persistently the fishing ground on which fewest sea lions were recorded during fishing operations in October, despite being the ground nearest the largest sea lion colony, Cobquecura (ca. 2500 animals). There are other potential explanations for sea lions being fewest at purse-seine operations closest to their largest haul-out colony, such as an effective, though untested, association of pinnipeds with other fisheries (e.g. bottom trawling for hake, M. gayi, and the coastal artisanal fishery for Patagonian toothfish, Dissostichus eleginoides). There could also be a seasonal pattern of sea lions on the fishing grounds, something not investigated in this study. Further, the results may be compromised by the fact that the number of sea lions at a purse-seine could be underestimated at night (Wickens, 1994), because deck lights are turned on only after a fish shoal has been encircled and net retrieval started, and only then can counting begin.
Our data suggest that, besides increased food availability as a direct effect of a fishery on sea lion behaviour, there is an indirect effect resulting simply from the presence of a fishing vessel in a given place. In fact, there was no significant difference between the number of sea lions observed at successful and unsuccessful sets or between the number at jack mackerel and whiptail hake sets. However, George-Nascimento et al. (1985) recorded M. magellanicus as the prey item with the highest Index of Relative Importance in the diet of the southern sea lion.
Judging from the number of sea lions observed as a function of the number of fishing vessels in an area, there appears to be a surfeit of potential food for them and no competition with the fishing fleet. This conclusion raises the question of the extent to which the sea lion population is engaged in this feeding behaviour.
With regard to the behaviour of sea lions at the purse-seine, some displays revealed a degree of specialization in collecting the fish entrapped in the purse-seine, with minimum investment of energy. The only apparent energy investments were on movement displays during the fishing operation and feeding displays mainly during the second half of the operation (at net retrieval). Consistent with the surfeit of food, resting dominated the displays through a fishing operation (e.g. upside-down vertical resting, with only hind flippers above the surface; and watching or surveying in a vertical position with head and neck above the surface). Even some of the movement displays could be associated with resting. Indeed, flat, steady swimming at the surface around the floatline and the other resting displays described earlier were concentrated during the early part of the operation.
With respect to the effects of sea lions on the fishery, Wickens (1994) pointed out that the main negative impact South African fur seals (Arctocephalus pusillus pusillus) have on purse-seine fishery catches was to cause a fish school to sound when it had been surrounded by the purse-seine net. In the Chilean jack mackerel fishery, we did not record any such impact on the fish school, neither dispersal nor sounding, nor could we find any skippers' references to school disturbance that could have been attributed to the presence of pinnipeds.
The Chilean jack mackerel fleet operates throughout the year, catches peaking during winter and summer, and declining during autumn and spring (the sampling period was in the austral spring). During 1999, the maximum monthly catch was 293 000 t in December and the minimum was 1070 t in February. The October catch was 25 000 t of jack mackerel and the mean monthly catch for 1999 was 100 000 t (SERNAPESCA official statistics).
The most obvious effect southern sea lions have on purse-seine catches is the direct consumption of entrapped fish, which amounted to 0.39% of the vessel's catch during the sampling period or 0.45% of the entire fleet's catch then. This estimate assumes first that sea lions are replaced in successive net-setting, and second that every individual consumes its entire daily ration in just one purse-seine set. In the case of the southern sea lion, part of the daily ration may be obtained by consuming whiptail hake because the industrial fleet catches both jack mackerel and whiptail hake. Moreover, operational consumption estimates would be more accurate if the maximum meal size were to be accounted for, but no original or literature data are available to allow for it. The values of daily ration of southern sea lion we used here, from George-Nascimento et al. (1985), are higher than others for the same species, such as for the Argentine Patagonian shelf (M. Koen-Alonso, pers. comm.) or from energy-based estimations (L.A.H., unpublished data). Therefore, the operational consumption of jack mackerel by southern sea lions calculated here (0.39% of the catch) may not be that accurate, but it is not likely to be much greater. In other words, its impact on harvests is minor compared with that in some small-scale fisheries and in fish farming (Aguayo and Maturana, 1973; Torres, 1979; George-Nascimento et al., 1985; Oporto et al., 1991; DOPPLER, 1998). Unfortunately, the portion of the population of southern sea lions interacting with the fishery is unknown sufficiently accurately to determine the real impact of sea lions on the fishery. Although it may seem small on the basis of the calculations here (for October 1999), any abrupt decrease in the jack mackerel fishery or a significant and sudden increase in the sea lion population could lead to a much greater impact.
Only 20 sea lions were caught or injured during the observed fishing operations (0.64 sea lions set–1). This is a small number considering the frequent presence at and involvement of sea lions during purse-seine operations. It is remarkable too that only juveniles were caught or injured, although it does perhaps show the inexperience of young sea lions in recognizing the risks of feeding at a purse-seine and delaying departure from the net. Bjørge et al. (2002) noted that young gray (Halichoerus grypus) and harbour (Phoca vitulina) seals are more vulnerable to incidental capture than older animals, perhaps associated with their curiosity and playful behaviour, but also possibly with their lesser physical strength and physiological response than adults.
The number of sea lions dead or seriously injured at a purse-seine was lower than expected, especially given the generally poor attitude of fishers towards sea lions. Indeed, it may be possible to preclude sea lion mortality totally if fishers cooperate to do so. Nevertheless, we are aware that the extent of incidental mortality may have been underestimated in our figures because the presence of external observers (researchers) may have altered the behaviour of the crew.
The values of fish consumption at fishing operations by southern sea lions calculated here are consistent with those given by Shaughnessy et al. (1981), Wickens et al. (1992), and Wickens (1994) for interactions between Cape fur seal and purse-seine fisheries off South Africa. Although pinnipeds apparently can influence fish aggregating behaviour, predation by otariids is not considered to be a serious problem for purse-seine fisheries (Shaughnessy et al., 1981), consistent with the current results.
The southern sea lion is a species found frequently in the coastal waters of Chile (Rivera, 1990), but it is found considerably further from shore off the Argentine Patagonian shelf (Campagna et al., 2001). The reason why sea lions move offshore more than 30 miles off Chile to feed on the fishing grounds of jack mackerel may be associated with learned opportunities for concentrated feeding, rather than having to locate disparate food sources directly (Shaughnessy et al., 1981). Fish caught in the net represent an easy and abundant food source that requires minimum metabolic expense compared with pursuing and catching prey individually. However, movement of sea lions from the coastal colonies to the offshore fishing grounds still demands greater metabolic expense than the swimming and thermoregulating associated with prolonged time in the water (Hind and Gurney, 1997). Information to address this apparent anomaly is lacking, however, so its solution remains an open question.
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Acknowledgements |
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We thank Peter Shaughnessy and Roger Kirkwood for their comments on the initial manuscript, and Simon Northridge, Jeremy David, and Andrew Payne for contributing suggestions that improved the manuscript greatly. We also thank the skipper and crew of the fishing vessel, and personnel from the local fishing industry for their support, and SERNAPESCA, S. Purca, S. Dans and M. Koen-Alonso for their cooperation in fulfilling the objectives of the work. L.A.H.'s graduate studies are supported by the Escuela de Graduados, Universidad de Concepción.
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References |
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