ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on July 21, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(6):1116-1123; doi:10.1093/icesjms/fsm082
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Underwater television as a fishery-independent method for stock assessment of Norway lobster (Nephrops norvegicus) in the central Adriatic Sea (Italy)
1 Istituto di Scienze Marine—CNR, Sede di Ancona, Largo Fiera della Pesca, 60125 Ancona, Italy
2 University Marine Biological Station Millport, Isle of Cumbrae KA28 0EG, UK
Correspondence to E. B. Morello: tel: +39 071 2078869; fax: +39 071 55313; e-mail: b.morello{at}ismar.cnr.it
Morello, E. B., Froglia, C., and Atkinson, R. J. A. 2007. Underwater television as a fishery-independent method for stock assessment of Norway lobster (Nephrops norvegicus) in the central Adriatic Sea (Italy). – ICES Journal of Marine Science, 64: 1116–1123.Norway lobster is of great commercial importance throughout the NE Atlantic and Mediterranean, where it lives in burrows in muddy sediments. The fact that the species is caught in commercial gear only when it emerges from its burrow and the absence of hard structures available for age determination complicate the application of normal fishery-dependent stock-assessment methodologies. This study provides more evidence of the usefulness of underwater television surveys as a fishery-independent technique to assess the Nephrops stocks of the Adriatic Sea. The results are compared with those of previous studies, and the advantages and disadvantages of using such methodology discussed in an Adriatic context.
Keywords: Adriatic Sea, burrows, fishery-independent assessment, Norway lobster, underwater television
Received 26 June 2006; accepted 11 May 2007; advance access publication 21 July 2007.
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Introduction |
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The Norway lobster (Nephrops norvegicus), hereafter referred to by genus alone, is of major commercial importance throughout the NE Atlantic and Mediterranean, including in the Adriatic Sea (Froglia, 1972; Chapman, 1980). In the Adriatic, Nephrops ranks first of all crustacean species exploited in terms of value, and second in terms of weight, with a decreasing trend in catches since 1993 (Vrgo
et al., 2004), accounting on average for one-third of the total Nephrops official landings in Italy. It is found on muddy grounds at depths from 
50 to >400 m (Artegiani et al., 1979), with important fishing grounds at 70 m off Ancona and at 220 m in the Pomo pit (IMBC, UMBSM, and IRPEM, 1994; Froglia and Gramitto, 1981, 1988; Froglia et al., 1997). With respect to Nephrops, Adriatic trawling grounds have been classified as fully exploited or overexploited (Sardà, 1998a). In particular, the topography, bottom-sediment composition (fine mud sloping down to 260 m), and oceanography of the study area, the western Pomo pit (central Adriatic Sea), combine to make it particularly important not only for Nephrops but also as the main nursery ground of the commercially important European hake (Merluccius merluccius) (Zupanovic and Jardas, 1986). Maximum trawling effort for Nephrops coincides with the presence of significant quantities of juvenile hake. For those reasons, the Pomo pit has been the subject of many discussions aimed at establishing it as an area closed to bottom trawling. In this context, careful management of the area and its main resources is vital, especially because two mixed-species trawling fleets from two different countries (Italy and Croatia) fish there regularly. Stock assessment of Nephrops is complicated by the fact that the species is caught in commercial gear only when it emerges from its burrow. Emergence varies with time of day, season, animal size, sex, and reproductive status, so the fishery exploits the population selectively and in a different manner according to sex (Froglia, 1972; Atkinson and Naylor, 1976; Naylor and Atkinson, 1976; Aréchiga et al., 1980; Chapman, 1980; Froglia and Gramitto, 1986; Tuck et al., 2000). Moreover, Nephrops lack hard structures bearing marks indicative of age, so the standard age-based methodologies applied in fishery-dependent stock assessment cannot be applied. Methods relying on length compositions have been applied, but they depend on reliable growth data, which are not always available. For these reasons, fishery-independent methods of stock assessment are of particular relevance to the species, and the most practical of these uses burrow counts as an index of stock abundance. The methodology for this involves the use of towed, underwater television (UWTV) or still photography. The former was initially developed in Scotland (Chapman, 1985; Bailey et al., 1993) and subsequently refined during several EC-funded international study projects in support of the Common Fisheries Policy (Marrs et al., 1996, 1998; Tuck et al., 1997b). The method is now used as standard in the UK (Marrs et al., 1998; Tuck et al., 1999), where more than 50% of the European catch of Nephrops is taken. The same method has also been used successfully in the Adriatic Sea in exploratory studies (e.g. Froglia et al., 1997), which are built on in the present work, and elsewhere in the Mediterranean (Smith et al., 2003). An approach using still photography has been developed in New Zealand for assessment of the similar species Metanephrops challengeri (Cryer et al., 2001), and that method has been used in the western Mediterranean for Nephrops (Aguzzi et al., 2004).
Usually, UWTV surveys provide an index of stock abundance that can be used to assess trends in stock status, but in other instances, and subject to certain assumptions, they can be used to provide stock-biomass estimates. This study attempts to do both through an evaluation of the Nephrops stock in the western Pomo pit (Adriatic Sea, Italy). Previous results obtained from the area (Froglia et al., 1997) are refined by improvements in the estimates of Nephrops biomass.
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Material and Methods |
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Data collection
The assessment was carried out on the offshore grounds of the western Pomo pit (42°52'N 14°57'E), central Adriatic Sea (Figure 1). The Nephrops stock in the area is characterized by high densities, but with a prevalence of small animals, compared with other populations in the Adriatic (e.g. off Ancona) (Froglia et al., 1997). At Pomo, catches peak at sunset and sunrise, and the lowest catches are recorded at night (Froglia et al., 1997). These factors were taken into account when planning the bottom-trawl tows (Table 1).
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Experimental work at sea was carried out in the period 17–21 August 2004 within a rectangle of 3 x 4 nautical miles at depths ranging between 210 and 235 m (Tables 1 and 2, Figure 1). A sledge-mounted UWTV was towed slowly (
1.5 knots) by CNR's RV "G. Dallaporta". The sledge used was constructed of aluminium and based on a design by Shand and Priestley (1999). The camera was a Kongsberg Simrad OE 1364 colour camera angled obliquely forwards to view the seabed. The field of view was 0.85 m wide at a horizontal plane of 40% of the screen height above the bottom edge of the TV monitor. Illumination was obtained by two 500W Versabeam lamps (Remote Ocean Systems, San Diego, USA). The camera cable was attached to the towing cable and deployed from the stern of the vessel. The camera control gear and TV monitor were viewed in the laboratory of the research vessel, and the visual output was recorded on videotape for subsequent analysis. A bridge record was kept of the vessel's position and speed, and this was used to compute the distance traversed by the UWTV sledge. This was then converted to area by multiplying it by the field width (0.85 m).
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Otter-trawl samples (stretched mesh 42 mm) were carried out in parallel with the UWTV survey between sunrise and sunset, coinciding with the times of maximum emergence from burrows, minimizing catch variability (Table 1). For comparative purposes, the Nephrops density "perceived" by the otter trawl was estimated: the distance covered by the net was calculated using positions of the start and end of the haul, then multiplied by the mean horizontal opening of the net [obtained from the SCANBAS SGM–15 system (SCANMAR, Norway) placed on the gear] to derive swept area. Nephrops caught by the trawls were used to compute the mean carapace length of the animals in the area. Mean length was then converted to mean weight using the length–weight relationship, determined for males and females combined (CF, unpublished): log10weight = (–3.5539) + 3.2510(log10 carapace length). Mean weight was used to obtain biomass estimates of Nephrops and to compare the results obtained in 2004 with those obtained for the same area in 1993 and 1994 (Froglia et al., 1997).
Assumptions
Certain assumptions underpin the methodology:
- It is assumed that the burrows were accurately ascribed to Nephrops. There are a number of burrow features that are species-specific. These relate to the shape and appearance of burrow openings, the size and angle of tunnels, and the geometric relationship between openings, plus features such as tracks next to the openings (Chapman and Rice, 1971; Atkinson, 1974; Chapman, 1980; Tuck et al., 1994; Marrs et al., 1996). Burrows of uncertain occupancy were not counted. Therefore, burrow counts are considered to be conservative.
- A cluster of openings judged to be related, based on the above criteria, was considered to represent a single burrow system. In other words, individual openings were not enumerated, but clusters of openings were interpreted to represent individual burrows (Bailey et al., 1993; Marrs et al., 1996).
- All burrows were assumed to be occupied. It is known that unoccupied burrows rapidly degrade and collapse (Marrs et al., 1996). Any burrows in a state of collapse or with partially blocked openings indicative of neglect were ignored in the counts.
- Each burrow was assumed to contain a single animal. This assumption has been validated in previous studies, subject to certain provisos. Juvenile Nephrops attach their burrows to those of adults (Chapman, 1980; Tuck et al., 1994), and the resulting burrow complexes are distinctive; in the current study, such burrows were counted as containing one animal. Justification for this is that very small Nephrops do not normally leave their burrows so do not constitute a significant component of the commercial catch. There are also occasions when two adults of the same size will compete for a burrow and co-occupy it until the dominance of one is established (Chapman and Rice, 1971), but such occurrences are not common.
Data analysis
This work involved looking through all videotape records and counting Nephrops burrows from selected timeframes of footage; a total of 470 min. Nephrops is one of many species that construct burrows on the grounds investigated, and it was therefore necessary to distinguish Nephrops burrows from those of other species. This was done based on the research experience with the burrowing fauna (e.g. Atkinson, 1986), and following protocols developed in previous studies (Marrs et al., 1996; Atkinson and Froglia, 2000). Burrow counts were made by two viewers.
A correction factor related to "edge effects" was applied to the data. Any count of burrows from the UWTV output will comprise those that are wholly in view plus those that extend from the field of view to adjacent unseen sea bed. Such burrows would be counted again if the UWTV were to be towed parallel to and abutting the first tow. Edge-effect quantification is important especially on grounds such as those investigated here, because of the high density of small animals. Studies carried out by Marrs et al. (1996) give little credence to the supposition that Nephrops burrows can be relied on to have a single main opening which would therefore not be counted in an adjacent tow. Even where there was a single main opening, a frequent occurrence, an adjacent tow would not have it as a reference, thus resulting in duplication of counts. On grounds such as Pomo, we consider an edge-effect correction to be important, especially because in fisheries management overestimation of a stock may be dangerous. Theoretical and empirical studies have shown that this edge effect will normally result in an overcount of burrows per UWTV tow of between 25% and 35%, the precise value depending on the size distribution of the burrows (Addison and Bell, 2000). The value increases with burrow size. If the burrow count is used solely as a comparative index of abundance, and the size range of burrows does not change, there is no need to apply an edge-effect correction. However, if the value is used for biomass estimation within a defined area, then the edge-effect correction factor needs to be applied. Here, sections of footage were analysed for edge effects, for a total of 37 min.
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Results |
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UWTV survey
Counts of Nephrops burrows in the western Pomo pit were taken from survey transects of a total of
15800 m of seabed, corresponding to a surveyed area of 13400 m2. Burrow densities of Nephrops in the area varied from around 0.40 burrows m–2 to some 1.24 burrows m–2, with a mean of 0.78 burrows m–2 (Table 2). The values obtained in 2004 were only slightly higher than those estimated for the same area in 1993 when mean Nephrops density was 0.69 burrows m–2 (s.d. 0.03 burrows m–2; Froglia et al., 1997).
On average, the analysis of edge effects resulted in an overcount of burrows of 25.6% (Table 3). These empirically determined values are in line with theoretical expectations for the small burrow sizes that dominated these grounds. Application of the edge-effect correction resulted in a mean estimated density of 0.58 burrows m–2 (range: 0.30–0.92 burrows m–2; Table 2). Although a relatively small proportion of the overall footage was submitted to edge-effect analysis (just 37 min), the fact that the results obtained were within the expected values (Addison and Bell, 2000), and the fact that the edge counts were consistent, led the authors to believe the correction factor to be representative of and applicable to the entire footage. Both the standard deviation of the mean overall edge correction (1.2%) and the mean standard deviation obtained when considering each minute analysed separately (3%) were small.
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Notes were also made of other burrowing species present on the grounds, identified from their distinctive burrow morphologies or, in some cases, sight of the burrow occupant. The main ones were the thalassinid shrimps Calocaris macandreae (abundant), Jaxea nocturna (common), and Callianassa subterranea (frequent), Fries' goby (Lesueurigobius friesii) (common), and various mound-builders including the echiuran Maxmuelleria gigas and an unidentified but distinctive burrow type. All these are separable from the burrows of Nephrops (Marrs et al., 1996; Atkinson and Froglia, 2000).
The grounds sampled appeared to have been heavily trawled, from the distinctive marks on the seabed made by trawl doors and ground gear (cf. Coggan et al., 2001). Many burrows were re-established following infilling by trawling, and the traces generated by recent excavations often confirmed the identity of burrow occupants.
Trawl survey
The otter trawl catches were rather low: just 40 kg of Nephrops were caught in 11 trawls of 1 h (Table 1). Similarly, the density of Nephrops perceived by the trawl (calculated using swept area) was low, varying considerably with time of day (Table 1). The size distributions were dominated by small animals, as demonstrated in Figure 2, which is a comparison of the results of the trawl catch in 1997 and 1998 (Wieczorek et al., 1999) with those obtained in 2004.
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The sex ratio was male-dominated (M/F = 1.61), and ovigerous (berried) females were scarce. This was expected, because berried females are present in the Adriatic Sea from early summer through to winter (Froglia and Gramitto, 1981) and, when berried, withdraw into their burrows to protect the eggs. Therefore, trawl samples of the female population may be biased. Berried females were not considered separately from non-berried ones.
Biomass estimates
The surface area considered for estimating biomass was that reported in Froglia et al. (1997) (Table 4). The density estimates used in the calculations were those taking edge effects into account (Table 2). An overall biomass of 7294 t of Nephrops was estimated for the western Pomo pit. This value is most likely an underestimate because the mean weight calculated from the trawl catches may have been affected by the low vulnerability of berried females attributable to the time of year the surveys were made. Berried females are larger than non-berried ones, and their inclusion in the calculation would have contributed to an estimated greater total biomass. Also, burrow counts were conservative (see above). Table 4 summarizes the results obtained from our study and compares them with those obtained in 1993 and 1994 (Froglia et al., 1997). The biomass estimated for the Pomo pit by the two studies is similar, and so are the mean Nephrops sizes reported.
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Discussion |
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At present, the assessment of Nephrops stocks in the Mediterranean relies almost exclusively on fishery-dependent techniques (Sardà et al., 1998). These methods are based on the use of trends in catch and catch rate which, for official statistics, can be unreliable, or on analytical methods such as virtual population analysis (VPA), length-cohort analysis, and yield-per-recruit analysis (Sardà et al., 1998; GFCM, 2002). The failings of these methods when applied to Nephrops stocks are several, because they rely on assumptions such as equal capture availability and stock redistribution following capture (Caddy, 1975), which do not hold for Nephrops. Moreover, because in the Adriatic Sea fishery statistics appear heavily underestimated and only recently has market sampling been implemented (R. (CE) 1543/2000), stock assessment of demersal resources relies mainly on trawl surveys (Sardà, 1998b; Lleonart and Maynou, 2003). Because Nephrops live within the sediment and their emergence is generally of short duration and associated with factors including time of the day, nutritional state, and reproductive state, methods based solely on trawl-survey data will be unreliable because only a small fraction of the population is considered. It is widely recognized in other parts of Europe (e.g. the North Sea and the NW Atlantic) that fishery-dependent methods need to be integrated with fishery-independent techniques for stock assessment (Bailey et al., 1993; Tuck et al. 1997b; Smith et al., 2003; Tuck and Bailey, 2004). This is particularly pertinent in the Adriatic Sea where the only stock-assessment technique used for demersal resources (trawl survey) appears to be inadequate for Nephrops, despite being a fishery-independent one. The difference between Nephrops densities estimated by trawl surveys and UWTV surveys is striking: two orders of magnitude smaller for trawl surveys (0.021 t km–2,
imunovi, 1997; 0.018 t km–2, Coll et al., in press; 0.034 t km–2, this study) compared with UWTV (6.11 t km–2, IMBC, UMBSM, and IRPEM, 1994; 5.93 t km–2, this study). The fact that Nephrops is an important fishery resource should drive the need to adopt adequate stock-assessment methodologies in the area.
The present work sought to develop UWTV methods as a tool for Nephrops stock assessment in the Adriatic Sea, with particular reference to one important Nephrops ground of the central Adriatic, the Pomo pit. We built on previous work carried out by Froglia et al. (1997) in the same area and improved the methodology through inclusion of an edge-effect correction, deemed useful in precluding the overestimation of densities. Moreover, re-application of this methodology 10 years later was considered important in view of the heavy fishing activity and declining catches reported for the area since 1993 (Vrgo et al., 2004). The UWTV survey revealed high densities of the species within the ranges reported for other Nephrops grounds in European waters assessed using the same methodology (western Irish Sea: Lordan et al., 2004; Aegean Sea: Smith et al., 2003; Firth of Clyde, Scotland: Tuck et al., 1997b; Scotland: Tuck et al., 2004).
The Pomo ground is an area where regional hydrography, low benthic biomass, recruitment of Nephrops postlarvae, and sedimentological factors combine to give a high density of small animals. It has been postulated that high levels of recruitment, combined with density-dependent growth, result in high Nephrops densities but poor growth rates on this ground (IMBC, UMBSM, and IRPEM, 1994), as in other areas, e.g. the south Clyde Sea area of Scotland (Tuck et al. 1997a; Parslow-Williams et al., 2000, 2002). This is consistent with the results presented here and data reported previously (Froglia et al., 1997; Wieczorek et al., 1999).
An attempt was made to estimate the total biomass of Nephrops in the study area, and the results were compared with those obtained previously (Froglia et al., 1997). The estimates obtained in 2004 were similar to those reported for 1993 and 1994, but it must be borne in mind that the findings reported then by Froglia et al. (1997) did not take into account edge effects. The densities were similar in both studies, as was mean size and therefore mean weight of individuals caught by the trawl. The Nephrops stock in the area appears, therefore, to have been maintained over the years despite the intense fishery operating in the area, and the UWTV survey technique appears to be particularly suited to these grounds. Unfortunately, the geographical extent of the present study is limited, compared with the size of the entire western Pomo pit. However, based on published knowledge of the Nephrops stock and the homogenous hydrographic and sediment characteristics in the pit, we suggest that results can reasonably be extrapolated to the whole area (IMBC, UMBSM, and IRPEM, 1994; imunovi, 1997). The classical stratified, random-sampling experimental design applied in UWTV surveys, with strata defined by sediment composition (Bailey et al., 1993), did not need to be applied because of its homogeneity (imunovi, 1997), and a distribution of the sampling stations in a random manner across the whole ground would have reduced the variance of the survey estimates. This is confirmed by comparison of the values of standard deviation for the density estimates obtained in 1993/1994 (0.03 individuals m–2, Froglia et al., 1997) and that obtained in this study (0.18 individuals m–2). Because of this, future work will evaluate differing survey designs.
Use of this methodology (UWTV survey plus a trawl survey) in future would allow the systematic assessment of the stock, employing comparatively limited additional effort and allowing year-to-year variation in biomass and size composition of the exploited population to be identified and monitored. Moreover, the fact that the Pomo pit is also the main nursery area for European hake in the Adriatic Sea (Zupanovic and Jardas, 1986) should act as a spur for careful assessment and management of its fishery resources in future.
Nevertheless, several problems are associated with this type of stock assessment, which could become important especially if the methodology were to be exported to other grounds with different stock characteristics (e.g. the Nephrops grounds off Ancona). Most importantly, the mean weight used to convert density estimates derived from UWTV burrow counts may not be representative of the fishable stock. Given the seasonal variability in Nephrops catch rates and sex ratios, sampling at a particular time could have considerable effects on mean size, and therefore on mean weight. Consequently, great care should be taken when using the mean size from trawl catches to raise density estimates to biomass: one may be counting the burrows of animals smaller than those caught, resulting in an overestimation of total biomass. This may become a problem when the Nephrops population is not as homogenous, in terms of animal size, as that now present in the Pomo pit. Determination of the representative mean weight of individuals on the grounds under investigation is essential if UWTV survey data are to be used successfully as a tool for stock assessment. Particular attention should be paid to carrying out trawl surveys during times when the probability of emergence is greatest for both sexes. Future studies should focus too on establishing a relationship between burrow size (using burrow-entrance diameter as a proxy) and occupant size. Owing to the depths at which Nephrops live in the Adriatic, which preclude the use of SCUBA, the UWTV system should be calibrated in such a manner as to allow measurements to be taken from the footage, and UWTV surveys should be conducted at times known to coincide with the maximum emergence of the species. Footage could then be analysed using image-analysis software.
This work provides further evidence of the usefulness and need of UWTV surveys for stock assessment of Nephrops in the Adriatic Sea. The results obtained underline the potential of the technique, pointing to the importance of incorporating this methodology into the existing assessment framework in the Adriatic Sea, which is currently inadequate for Nephrops. Further, more detailed studies will follow on other Nephrops grounds, taking into account the problems mentioned above.
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Acknowledgements |
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We thank Bruno Antolini and the skipper and crew of RV "G. Dallaporta" for their help at sea, and Nick Bailey, Ian Tuck, and Enrico Arneri for useful comments on the manuscript. The study was carried out with financial support from the European Commission, SSP8-CT-2003-501605, NECESSITY. This paper does not necessarily reflect the views of the European Commission and in no way anticipates future opinion of the Commission. RJAA acknowledges travel and subsistence support from the Consiglio Nazionale delle Ricerche (CNR).
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