Simulating escapes of farmed sea bass from

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Simulating escapes of farmed sea bass from

Mediterranean open sea-cages: Low recaptures by local fishermen

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Short communication

Simulating escapes of farmed sea bass from Mediterranean open sea-cages: low recaptures by local fishermen

By P. Arechavala-Lopez, D. Izquierdo-Gomez, P. Sanchez-Jerez and J. T. Bayle-Sempere Department of Marine Sciences and Applied Biology, University of Alicante, Alicante, Spain

Introduction

During the past years, farmed European sea bassDicentrar- chus labrax (Linnaeus, 1758) escapees have been recorded around farms in different areas (for a review see Sanchez- Jerez et al., 2011). However, there is a lack of knowledge about their dispersal and survival capabilities. A previous study on escaped sea bass from Mediterranean farms demon- strated their ability to survive for up to 3 weeks in the wild, moving quickly and repeatedly between and among several farms (Arechavala-Lopez et al., 2011), suggesting a risk of pathogen transmissions to nearby farmed and wild fish stocks (Arechavala-Lopez et al., 2011, 2013). Escaped sea bass could also lead to other potential ecological risks through predation and resource competition with wild populations, as already reported for escaped sea bream (Arechav- ala-Lopez et al., 2012). The aim of this study was therefore to provide detailed evidence of whether sea bass escapees are long- or short-lived, as well as to assess the habitat use and feeding habits of externally-tagged sea bass recaptures by local fishermen, following simulated escapes from open-sea cages in the Mediterranean Sea.

Materials and methods

The study was carried out in a Mediterranean coastal farming area situated in the southeast of Spain (Fig. 1). Three ﬁsh farm facilities rearing sea bream and sea bass (F1, F3

and F₄), plus another farm (F₂) that only presented floating rings and anchored structures, are located in this area (Fig. 1). All farms are located 3–4 km offshore over soft muddy bottoms in depths ranging from 23 to 30 m (for fur- ther details see Arechavala-Lopez et al., 2011, 2012). A total of 1186 sea bass individuals were externally tagged and released at the same farm growth facility (R; see Fig. 1) to simulate escapes at two different times on the same day. Spe- cifically, 688 and 498 sea bass individuals were tagged and released on 29 July 2011. Average length (standard length SD) and average weight (total weight SD) at the time of release were 26.41.8 cm and 302.981.6 g, respectively (calculated from a subsample of 12% of total tagged fish).

T-bar anchor tags (Hallprint Ltd., Victor Harbour, SA, Australia), each with an individual code and telephone number to contact in the case of recapture, were inserted into the flesh, about 1 cm behind the base of the dorsal fin. This type of tag allows the fish freedom of movement, is relatively quick and easy to apply, and has good retention properties (Pawson et al., 1987). A similar tagging method has also

been successfully applied in previous studies on wild and hatchery-reared sea bass (Pickett et al., 2004; Pawson et al., 2007; Grati et al., 2011). Information on the tagging pro- gramme (poster and pamphlets) was sent to recreational and professional fishermen’s associations, ports, fishing stores and fishery research centres, covering an area of more than 30 km from the farming area. When a tagged fish was recaptured, the fishermen provided the fish, information on the identification code, time of capture, fishing method and the recapture point. Moreover, standard length (cm) and total weight (g) were recorded from recaptured tagged sea bass individuals, which were immediately frozen after capture.

Stomach contents of the recaptured sea bass were dissected and preserved in 70% ethanol. Gut contents were observed under a microscope and dietary items were counted and weighed after the removal of surface water with blotting paper. The same process was carried out on 50 wild sea bass individuals (average standard length SD: 25.21.8 cm, total weight: 266.6 59.3 g) captured by local artisanal ﬁsh- eries, in order to assess the feeding behaviour of the wild sea bass population of similar size in the study area.

Results

Altogether 15 D. labrax individuals (1.26% of total tagged and released sea bass) were recaptured following the simulated escape incidents (Table 1). All were caught at 4 km distance from the release cage and, gradually, over 3 months by local recreational anglers within an estuarine environment formed by the Segura River, at the entrance of Guardamar harbour (C; see Fig. 1). Professional fishermen reported no recaptures at the mouth of the river. The first recapture was on day 7 after release; the last recapture was 94 days after release (Table 1). From the 15 sea bass recaptured by local anglers, feeding habit analyses were only possible for the nine specimens that were recovered intact (Table 1); four of these nine recaptured D. labrax had empty stomachs (44.5%). Individual #4, recaptured 13 days after release, had five decapod larvae in its stomach (Table 1). Particulate organic matter, similar to the river bottom, was found in the stomach of sea bass #6, which was recaptured 24 days after release. Moreover, food pellets were found in the stomach of individual #11, which was in the wild for 37 days. Sea bass

#13 and #14, recaptured 44 and 69 days after release, respectively, had ﬁsh remains and polychaeta, respectively, in their stomachs. Regarding the feeding habits of the wild sea bass, a total of 28 individuals (56%) had empty stomachs. In J. Appl. Ichthyol. (2013), 1–4

Received: June 26, 2013 Accepted: August 14, 2013 doi: 10.1111/jai.12357

Applied Ichthyology

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terms of relative abundance, the main prey items found in wild sea bass were benthic ﬁsh (mainly gobiidae, 44%), fol- lowed by crustacean decapods (shrimps, 37%) and ﬁsh remains (16%). Nevertheless, particulate organic matter from the sediment (7%), bivalves (4%), and isopods (4%) were also found with remarkable occurrence.

Discussion

Low numbers of tagged sea bass individuals were recaptured by local ﬁshermen (anglers) in the mouth of Segura River, a coastal recreational ﬁshing area located 4 km distant from the farm facilities. Similar recapture percentages were found in a previous study on hatchery-reared sea bass released in the Adriatic Sea (0.45% by Grati et al., 2011), and in mark- recapture studies on wild sea bass along the European Atlan- tic coast (3.7% by Pickett et al., 2004; 1.1; % by Fritsch

et al., 2007; and 4.5% by Pawson et al., 2007). However, they reported that the recaptured sea bass were caught using gillnets, trammel nets and ﬁshing rods in both in-shore and oﬀ-shore waters (Pickett et al., 2004; Fritsch et al., 2007;

Pawson et al., 2007; Grati et al., 2011). In the present study, the resulting recaptures suggest diﬀerent scenarios. On the one hand, escaped sea bass might present high natural mortality rates after release, as suggested by other authors (Toledo-Guedes et al., 2009; Arechavala-Lopez et al., 2011).

Natural mortality through predation appears to be important in reducing the number of escaped sea bass immediately after escape. Handelsman et al. (2010) reported relatively poorer sprint performances and lower survival rates of cultured sea bass compared to wild conspecifics against avian predation in mesocosms, suggesting that cultured sea bass would not fare well in a natural environment. In fact, predator species such as large finfish and fish-eating birds are attracted to fish farms but are also commonly present in coastal and estuarine environments (e.g. Melotti et al., 1994, 1996; Beveridge, 2001; Sanchez-Jerez et al., 2008). On the other hand, external tags can be lost, but also fishermen might prefer to keep the fish despite their being aware of the mark-recapture pro- gramme. Thus, the recapture rates should be regarded as minimum estimates (Sanchez-Lamadrid, 2001). Finally, the possibility cannot be ruled out that sea bass escapees swam away from the study area, either along the coast or to deeper waters, where they are more difficult to capture. Neverthe- less, all recaptured individuals were caught in the mouth of the river, which flows into the sea via a small harbour. This fact demonstrates that some escaped sea bass must have left the farm facilities to move towards shallower waters inshore.

This behaviour was also observed in previous studies where either escaped or hatchery-reared sea bass were commonly found in shallow coastal habitats, mainly in harbours, mari- nas or estuaries (Toledo-Guedes et al., 2009; Grati et al., 2011).

Regarding stomach content analysis, it was evident that farmed sea bass were able to forage on common natural prey (i.e. polychaeta, decapod juveniles, ﬁsh remains) once they escaped from a farm. Although these data are from a limited number of specimens, the results are in accordance

Table 1

Time of recapture (date and days after release), distance between release farm (R) and capture point (C), fishing gear used by local fishermen, standard length (SL), total weight (TW) and stomach contents of recaptured tagged sea bass during the study period.–=only the external tag recovered (no fish individuals)

Fish Id #

Capture date (day/month/year)

Days after

release Distance R–C (km)

Fishing

gear SL (cm) TW (g) Stomach contents

1 05/08/2011 7 4 Angler 26.2 372 Empty

2 09/08/2011 11 4 Angler 31.1 432 Empty

3 09/08/2011 11 4 Angler – – –

4 11/08/2011 13 4 Angler 28.5 430 Decapod juveniles (n=5; 0.3 g)

5 12/08/2011 14 4 Angler 28.5 338 Empty

6 22/08/2011 24 4 Angler 25.5 270 Particulate organic matter (0.6 g)

7 26/08/2011 28 4 Angler – – –

8 26/08/2011 28 4 Angler – – –

9 26/08/2011 28 4 Angler – – –

10 03/09/2011 36 4 Angler – – –

11 04/09/2011 37 4 Angler 27 305 Food pellets (3 g)

12 06/09/2011 39 4 Angler 29 450 Empty

13 11/09/2011 44 4 Angler 30.4 480 Fish remains (0.9 g)

14 06/10/2011 69 4 Angler 27 276 Polychaeta (0.1 g)

15 31/10/2011 94 4 Angler – – –

Fig. 1. Map of ﬁsh farming study area, Western Mediterranean. F:

ﬁsh-farm facilities. Tagged ﬁsh were released at farm F3 (Release site: R). Recaptures were caught at the mouth of the Segura River (Recapture area: C)

2 P. Arechavala-Lopez et al.

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with studied wild specimens, as well as with previous studies on wild and released hatchery-reared sea bass (e.g. in estuarine and lagoonal Mediterranean regions: Ktari et al., 1978;

Barnabe, 1980; Ferrari and Chieregato, 1981; Roblin and Brusle, 1984; Kara and Derbal, 1996; Rogdakis et al., 2010). Moreover, a wide variety of finfish were reported as preferred main prey for escaped sea bass in the Canary Islands, but also crustaceans were common as secondary prey (Gonzalez-Lorenzo et al., 2005; Toledo-Guedes et al., 2009). The presence of particulate organic matters in the stomachs could be indicative of the aggressive feeding behaviour of escaped sea bass, switching to prey on soft bottoms. Moreover, food pellets in the stomach could reveal a high affinity of escapees to the farming areas. However, Arechavala-Lopez et al. (2011) showed that sea bass escapees were observed at farms only during the first week after escape or release. On the other hand, recreational anglers very frequently use food pellets for baiting fish. The propor- tions of empty stomachs found in recaptured and wild sea bass were similar (44.4 and 56%, respectively). These percentages agree with other studies on escaped sea bass in the Canary Islands (66% in Gonzalez-Lorenzo et al., 2005; 50;

% in Toledo-Guedes et al., 2009) and on wild sea bass from Atlantic and Mediterranean populations (45% in Kelley, 1987; 34;.5% in Rogdakis et al., 2010; 33; % in Grati et al., 2011). Therefore, our results conﬁrm that escaped sea bass in the Mediterranean can be considered as an opportunistic species, clearly able to exploit natural resources. However, it seems that they might need time to adapt to new environments after net-pen escapes.

Reliable knowledge about coastal dispersion and feeding habits of farmed sea bass following a small-scale escape inci- dent in a coastal Mediterranean farming area has been shown in our study. Although there it is highly uncertain about survival of the sea bass escapees, it was proven that some escaped sea bass were able to survive for up to 3 months in the wild, moving to estuarine areas and switching to natural prey. These results might indicate a potential risk of escaped sea bass on wild assemblages through competition for natural resources and predation on other species, which could lead to ecological problems particularly when both massive escape and/or large-size ﬁsh escape incidents occur near coastal and estuarine populations. Additional knowledge regarding survival and movements of escapees following large-scale escape incidents is recessary to evaluate these potentially negative impacts, as well as to improve the management of escapee recapture and thereby reduce the risk potential of the species intermingling with natural populations.

Acknowledgements

We would like to thank the staff of the fish farms (Culmar S.L. Grupo Marjal) for their help and assistance in the field.

The study was ﬁnanced by the EU-project “Prevent Escape”

(Project Nr: 226885 http://www.preventescape.eu/).

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Author’s address: Pablo Arechavala-Lopez, Department of Marine Science and Applied Biology, University of Alicante, Ap. 99, E-03080 Alicante, Spain.

E-mail: [email protected]

4 P. Arechavala-Lopez et al.