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BY

J. AGUZZI1,2),A.BOZZANO1) and F. SARDÀ1) 1) Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, E-08003 Barcelona, Spain

ABSTRACT The density of Nephrops norvegicus burrows on the lower continental shelf (100-120 m depth) off the Catalan coast (western Mediterranean) was estimated by underwater photography. A total of 68 burrows were identified in 84 m2, equivalent to 0.81 burrows per m2. The conversion of this value to km2 gave an estimate of 81 × 104 burrows in the study area. To obtain the approximate number of performing emergence on a diel basis, density estimates were compared with those obtained over 24 h in a previous trawl survey, carried out in the same area at days of maximum and minimum catches. Findings are discussed taking into account already available data for the northeastern Atlantic, the Adriatic Sea, and the Aegean Sea.

RESUMEN Se estimó la densidad de madrigueras de Nephrops norvegicus en la plataforma continental (100-120 m) de la costa catalana (Mediterráneo oeste) mediante el uso de fotografía submarina. Se contaron un total de 68 madrigueras por 84 m2 de superficie muestreada, lo cual equivale a 0.81 agujero por m2. La extrapolación a km2 da un total de 81 × 104 madrigueras en el área de estudio. Con el fin de obtener el número aproximado de animales que realizan emergencia de la madriguera con un ritmo circadiario, se compararon las densidades de madrigueras con la densidad de individuos, obtenida ésta última sobre las capturas máximas y mínimas de muestreos realizados anteriormente con un arte de arrastre, en la misma zona y para un intervalo de 24 h. Los resultados se discuten teniendo en cuenta la información disponible de densidades de madrigueras del Atlántico, Adriático y Egeo.

INTRODUCTION To study the population dynamics of commercially important helps to evaluate their state of exploitation by considering density and capture indices. In

2) Fax: +34.932309555; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2004 Crustaceana 77 (3): 299-310 Also available online: www.brill.nl 300 J. AGUZZI, A. BOZZANO & F. SARDÀ the majority of cases, a homogeneous distribution of the species stock is assumed in the study area (Sparre et al., 1989). For this reason, different sampling systems may be used to evaluate stock demography. For the Norway lobster, Nephrops norvegicus (L., 1758), a species of high commercial importance in both the northeastern Atlantic and the Mediterranean, the sampling systems most often used are trawl sampling and direct underwater observations, such as, e.g., photography and video, for counting the numbers of burrows. Underwater techniques have been widely employed to directly estimate the burrow density of N. norvegicus on the English and Irish shelves. In these areas, towed sledges with videocamera systems (e.g., Chapman & Rice, 1971; Chapman, 1985; Tuck et al., 1997b; Pinn & Robertson, 2001), as well as direct counting by SCUBA divers (Chapman, 1979; Tuck et al., 1994), have allowed to define several ecological aspects of the burrowing life style of this species. Consequently, a lot of information is currently available in the literature on internal burrow architecture, entrance morphology, and the relative position of different burrow systems (e.g., Rice & Chapman, 1971; Farmer, 1975; Chapman, 1980; Atkinson & Chapman, 1984; Tuck et al., 1994; Smith et al., 2003). Animals inhabit single burrow systems, where a crater-like entrance opens into a tunnel running to a depth of almost 20 cm parallel to the bottom surface. The tunnels end in a variable number of perpendicular ventilator shafts, that have a greatly reduced cross-section (Farmer, 1974; Chapman et al., 1975). However, direct counting of N. norvegicus burrows still presents an unsolved problem, since data on density estimates obtained with underwater techniques give much higher counts than those recorded by trawl tow surveys. Chapman (1980) compared different methods of sampling on the English and Irish shelves (i.e., trawl catches and underwater television observations), and concluded that only 10-30% of the animals in a population emerge on a diel basis and are, therefore, vulnerable to capture by bottom trawl nets. Moreover, direct underwater observation has suggested that burrows may persist in the absence of their occupant for an undetermined length of time, due to the very low hydrodynamism typical of Nephrops inhabited bottoms (Hugens & Atkinson, 1997; Tuck et al., 1997b) or to occupancy by individuals of other species (Chapman & Rice, 1971; Atkinson et al., 1997). Only a few studies are available on N. norvegicus burrows in the Mediterranean basin and all of these refer to Adriatic and Aegean Sea populations (Froglia et al., 1997; Smith et al., 2003; Smith & Papadopoulou, 2003). No data are available, to date, on burrow density estimates of western Mediterranean populations. However, a recent field survey carried out off the Catalan coast (Aguzzi et al., 2003) proposed reliable density estimates for lower shelf (100-120 m) animals, since it assessed the population emergence rhythmicity over consecutive days. In fact, emergence NEPHROPS NORVEGICUS BURROW DENSITIES 301 rhythms affect stock dimension assessment by trawling, since the quantity of animals available to the nets depends on the time at which the tow occurs. As a consequence, the present study performed an underwater photographic survey along a transect selected in the same area as that of the previous field survey, in order to obtain density estimates from direct burrow counting for comparison with those obtained at days of maximum catches (Tuck et al., 1997b). Therefore, the aim of the present study was to analyse the accuracy of the direct counting of burrows in comparison with data obtained by trawl catches and to take this method into account for evaluating the size of the exploited local stock.

MATERIAL AND METHODS

The photographic survey was performed on board the research vessel “García del Cid” (38 m length; 1,200 hp), during October 1999, on the continental shelf (100-120 m) off the Ebro Delta, in the western Mediterranean (fig. 1). The study was performed in an area where substrate homogeneity (silt and clay fraction between 50 and 90%) allows the presence of a high Nephrops norvegicus density,

Fig. 1. Map illustrating: a, the locations of underwater camera deployments; and b, trawl surveys performed during October 1999. 302 J. AGUZZI, A. BOZZANO & F. SARDÀ according to the geostatistic studies of Maynou & Sardà (1997) and Maynou et al. (1998). Images were obtained by means of the benthos photographic system SRS 3000. A Nikon 801 camera (50 mm lens, aperture 2.8) was introduced in a pressure and water resistant stainless steel cylinder, together with a 9.5-volt battery pack and a flash unit with a maximum power rating of 35 O-J. The camera was equipped with a bottom switch connected to a cable with a weight, allowing the camera and flash system to operate at 2 m above the bottom surface. The camera was vertically oriented with respect to the bottom by means of a weight-balanced metallic supporting structure. The visual field recorded by the camera system was previously determined on board by taking a picture at two metres above the boat’s bridge, and corresponded to 4 m2. The pictures with the best resolution were selected for burrow identification and counting. The burrows of N. norvegicus were identified by following the description already available in the literature and crater-like front openings were used as markers of burrow entrances, defining different burrow systems (e.g., Chapman, 1985; Tuck et al., 1994). All burrows counted on each picture were averaged in order to obtain a mean density estimate. In order to compare this estimate with the one obtained by catches (Aguzzi et al., 2003), the total mean number of burrows was converted to the spatial scale of km2. Trawl catch data, recorded in the same area during field surveys of October 1999 and June 2000 (Aguzzi et al., 2003) were used as a reliable comparison of density estimates inferred by burrow counting. Catch data were recorded over 4 days of consecutive trawl tow replicates in the photographed transect area. These data were summed over 24 hours, in order to get an indication of the number of emerging individuals over in a complete diel cycle. The days with maximum and minimum catches were chosen as companions for burrow counting. Subdividing the number of burrows by the number of sampled animals over 24 hours at the day of maximum and minimum catches provided a relative ratio. In order to reduce bias in underwater and catch data comparison, i.e., to make reliable inferences on population demography, the present study as well as the previous catch surveys of October 1999 and June 2000 (Aguzzi et al., 2003), were both performed in the same area, where a maximum density and homogeneous distribution were previously established by Maynou et al. (1998).

RESULTS A total of 80 pictures were obtained and 21 were selected for the present analysis; details are presented in table I. The discrepancy in the number of pictures NEPHROPS NORVEGICUS BURROW DENSITIES 303

TABLE I Details of underwater camera deployments in which photographic images were recorded ◦ N of image Coordinates NE ◦   ◦   14038 26 1 02 58 ◦   ◦   24038 70 1 02 92 ◦   ◦   34039 45 1 03 55 ◦   ◦   44039 47 1 11 97 ◦   ◦   54039 82 1 10 05 ◦   ◦   64039 85 1 03 88 ◦   ◦   74039 95 1 11 86 ◦   ◦   84040 60 1 12 29 ◦   ◦   94040 63 1 08 32 ◦   ◦   10 40 40 87 1 04 95 ◦   ◦   11 40 41 31 1 12 01 ◦   ◦   12 40 42 28 1 14 97 ◦   ◦   13 40 42 47 1 11 63 ◦   ◦   14 40 43 09 1 15 78 ◦   ◦   15 40 43 27 1 12 91 ◦   ◦   16 40 44 06 1 16 41 ◦   ◦   17 40 44 10 1 14 24 ◦   ◦   18 40 44 56 1 14 96 ◦   ◦   19 40 45 37 1 16 33 ◦   ◦   20 40 45 79 1 17 03 ◦   ◦   21 40 46 29 1 17 76 taken with respect to those effectively used for burrow counting was chiefly due to bad weather conditions, which impeded correct camera positioning on the bottom surface when the boat drifted, and which also resulted in turbidity near the bottom. The majority of the pictures showed dense marks (i.e., holes, rises, and grooves) on the bottom surface, indicating a consistent bioturbating activity of the species constituting the benthic community in the transect area. All pictures revealed a complex set of burrow systems of different types. Although this fact led to difficulties in the identification of individual Nephrops norvegicus burrow systems, their entrances appeared with the usual prominent, crater-like feature (fig. 2; cf. Rice & Chapman, 1971; Chapman 1985; Tuck et al., 1994). No was seen out of its burrow on any of the pictures. A total of 68 burrows were identified in 84 m2, equivalent to 0.81 burrows per m2. The conversion of this value to km2 gave an estimate of 81 × 104 burrows per km2. In this area, Aguzzi et al. (2003) recorded a maximum in the catches during the first day of survey with 1906 individuals per km2, in June (see fig. 3a). The ratio between N. norvegicus burrows counted by underwater photography and animals sampled in catches per km2 was nearly 424. In other words, in June (when 304 J. AGUZZI, A. BOZZANO & F. SARDÀ

Fig. 2. Details of two underwater pictures showing the prominent, crater-like entrances of Nephrops norvegicus (L., 1758) burrow systems. The arrows mark burrow entrance and direction. Scales are 7cm(top)and5cm(bottom). maximum catches occur) there were 424 burrows for each animal caught by trawl nets over a 24-h period. In October, a minimum in the catches over 24-h period was recorded in the same area during the second day of survey, with 477 individuals per km2 (see fig. 3b), hence, the ratio between the numbers of counted burrows and captured animals per km2 was nearly 1698. Comparison of both ratio values shows marked seasonal differences, catches being higher in June than in October (Aguzzi et al., 2003).

DISCUSSION

In the present study, direct burrow counting by underwater photography pro- vided a density estimate of Nephrops norvegicus inhabiting the shelf off the Cata- NEPHROPS NORVEGICUS BURROW DENSITIES 305

Fig. 3. Time series of catches reported in the present area during a day of maximum (a, June) and minimum (b, October) captures (Aguzzi et al., 2003), used as a comparison for the direct burrow counts. lan coast. An estimate of 0.81 burrows per m2 (81 × 104 burrows per km2) was ob- tained and the results fit with those already reported for other areas of the Mediter- ranean Sea and the northeastern Atlantic (table II). In particular, the values ob- tained for the are similar to the estimates calculated by Froglia et al. (1997) in two areas of the Adriatic (table II). To the contrary, the present estimates are higher than those proposed by Smith et al. (2003) and Smith & Pa- padopoulou (2003) for the Aegean Sea (table II). Stock assessment from burrow counting should take into account the emergence rhythmicity of the species. In the present study, 81 × 104 burrows were counted per km2. When the equivalence 1 burrow : 1 animal is assumed (i.e., 100% occu- pancy rate; Tuck et al., 1997b), the percentage of animals performing emergence over a 24-h cycle spans from 0.05% in October (minimum in catches) to 0.23% in 306 J. AGUZZI, A. BOZZANO & F. SARDÀ

TABLE II Density of Nephrops norvegicus (L., 1758) as recorded by direct counting of burrows using SCUBA diving, underwater television (UWTV) or photographic (Pho) surveys in the Scottish (Sc), Irish (Ir), western Mediterranean (WM), and Aegean sea (AS), as part of the eastern Mediterranean; Ns, depth not specified

Source of data Study area Depth System Density (m) employed (no. of burrows/m2) Chapman & Rice (1971) Loch Torridon (Sc) 30 UWTV; SCUBA 2.1 Rice & Chapman (1971) Loch Torridon (Sc) 30 SCUBA 2.5 Hillis (1974) Dublin Bay (Ir) 5-10 SCUBA 1.0-2.5 Atkinson (1974b) Loch Aline (Sc) 10-40 SCUBA 0.15-0.16 Chapman (1979) Sound of Jura (Sc) Ns UWTV; SCUBA; Pho 3.0-5.0 Firth of Forth (Sc) Ns 0.4 Clyde (Sc) Ns 0.9-1.4 Linn of Morven (Sc) Ns 8.0 Chapman (1985) Linn of Morven (Sc) 75-100 UWTV; Pho 0.15-0.31 Tuck et al. (1994) Loch Sween (Sc) Ns SCUBA 0.092 Tuck et al. (1997b) Firth of Forth (Sc) Ns UWTV 0.46-1.48 Hugens & Atkinson (1997) Cumbrian Coasts (Ir) 30-40 UWTV 0.1-1.3 Froglia et al. (1997) Ancona (WM) 53-87 UWTV 0.11-0.63 Pomo Pit (WM) 213-240 0.66-0.72 Pinn & Robertson (1998) NE Isle of Sky (Sc) Ns UWTV 0.13-0.39 Pinn & Robertson (2001) Firth of Forth (Sc) Ns UWTV 0.48 Moray Firth (Sc) Ns 0.26 North Minch (Sc) Ns 0.27 Smith et al. (2003) Mytilini (AS) 360 UWTV 0.045 Limnos (AS) 280 0.025 Thasos (AS) 420 0.012 Skyros (AS) 380 0.018 Skiathos (AS) 440 0.017 Pagasitikos (AS) 80 0.120 Evoikos (AS) 450 0.003 Smith & Papadopoulou Pagasitikos (AS) 66-92 UWTV 0.007-0.166 (2003) Aguzzi et al. western 100-120 Pho 0.81 (present results) Mediterranean

June (maximum in catches), all others being apparently protected from the trawl nets (or escaping). The consistently lower number of animals captured in nets compared to burrow counts was explained by Chapman et al. (1975) and Chapman (1980) for the northeastern Atlantic shelf populations by assuming that only 10-30% emerged on a diel basis. Tuck et al. (1997a) suggested that emergence is affected by demography, being higher in high density areas, since animals perform longer foraging excursions. In fact, in the present study, percentages of emerging animals NEPHROPS NORVEGICUS BURROW DENSITIES 307

(inferred by comparing total 24-h catches with direct burrow counting) are much lower than those proposed by Chapman (1980) in the Atlantic (i.e., 10-30%), the density of Mediterranean shelves thus being consistently lower than the Atlantic (Farmer, 1975). Additionally, percentages of emerging animals vary as a consequence of seasonal fluctuations in light intensity at the bottom, catches being higher in spring-summer than in autumn-winter (Aguzzi et al., 2003). Apparently, N. norvegicus may or may not emerge on a diel basis, depending on its nutritional state in relation to its recent feeding history and to food availability (Oakley, 1979; Aguzzi, 2002). Alternatively, other factors, such as the reproductive cycle, may account for the disparity between the number of animals captured and the number of burrows recorded. Aguzzi (2002) reported a significant decrease in the number of hauled females in the western Mediterranean for October, which has been explained already in the literature as a result of an inhibition of emergence behaviour during the egg-bearing season (autumn-winter) (Sardà & Lleonart, 1993; Sardà, 1995; Orsi-Relini et al., 1998). Comparison between burrow density and the number of animals caught, as calculated by a trawl survey in the same area during the same period (Aguzzi et al., 2003), showed a marked discrepancy. On the other hand, Sardà & Lleonart (1993) and Maynou et al. (1998) applied length cohort analysis (LCA) to assess the N. norvegicus population in the same area. These studies incorporated several biological indicators such as sex-ratio, size-age relationship, class-size frequency distribution, and fecundity, and the estimated density matched those calculated by catches. Taking into account that direct estimates of burrow density should be considered reliable measurements of animal densities (Tuck et al., 1997b), both trawl catches and analytical methods consistently underestimate N. norvegicus density. Nevertheless, when direct burrow counting is employed to estimate population density, various sources for methodological bias are at issue. First, the identifi- cation of N. norvegicus burrows is not an easy process, especially in those areas where the bottom shows a high density of all kinds of marks due to bioturbation. Second, the sampling window may influence the density estimate when data are derived from areas of different substrate typology. In addition, burrow occupancy rate also represents an important source of bias in stock assessment studies using direct counting, which leads to uncertainty in the equivalence of 1 burrow : 1 animal. The occupancy rate is difficult to determine for two main reasons. First, uncertainties exist for the time of persistence of unoccupied burrows (Hugens & Atkinson, 1997; Tuck et al., 1997b). Second, other species sharing the same area, such as rhomboides (Linnaeus, 1758), may have burrow entrances with a similar architecture (Chapman & Rice, 1971; Atkinson, 1974a). Also, other opportunistic species, such as gobiids, may occupy 308 J. AGUZZI, A. BOZZANO & F. SARDÀ the abandoned burrows of this species while keeping their architecture unaltered (Chapman & Rice, 1971). Other behavioural traits of Nephrops norvegicus may represent a bias in density estimates as well, both in burrow counting and in catches. Apparently, the majority of juveniles form adult-juvenile burrow complexes in the field, but show a preference to inhabit existing burrows of adults in aquaria, even when occupied (Tuck et al., 1994). Juveniles are less easily caught, probably due to low out- of-burrow activity (Gramitto, 1998). Nevertheless, Aguzzi et al. (2003) recently showed marked emergence rhythmicity for at least a percentage of the juveniles, suggesting that net mesh size plays an important role in determining emergence rhythmicity of the different size-classes when using trawl catches. In conclusion, the present data suggest that any study focusing on Nephrops norvegicus stock density estimation is biased when only a single sampling method is employed. Methodological factors, as well as the peculiar life history traits of this species in relation to its burrowing habit, need to be taken carefully into consideration for stock density estimation purposes. The large difference recorded between density estimates from direct burrow counting and from catches is consistent with the presumed, apparent sampling inaccessibility of important components of the population, such as juveniles and berried females. More data on the real percentage of animals in the population that perform emergence on a diel basis, are needed. The inferred sampling inaccessibility of considerable parts of the population may, however, contribute to maintenance of the condition of the stock despite the intense fishing pressure during the last decade (Sardà & Lleonart, 1993).

ACKNOWLEDGEMENTS

The present work was undertaken in the context of a national research project funded by the Spanish Ministry of Research (CICYT) (NERIT: MAR98-0935). The authors also want to thank the crew of the R/V “García del Cid” (CSIC) for their support during sampling. Special thanks are also due to Mr A. Julià and Mrs M. I. Lloret for their valuable technical support and to Dr C. Rodgers for reviewing the final English version.

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First received 12 September 2003. Final version accepted 19 October 2003.