Resource use by common artisanal gear
The artisanal shrimp fishery along the western coast of Sri Lanka is complex, due to the multi-gear and multi-species context of both shrimp and fish. Compositions of shrimps caught with different types of gear seem to be a result of both gear selectivity and species' behavioural characteristics. Nine of 13 shrimp species spawn in coastal waters and have an essential estuarine phase to complete their life cycles; P. coromandelica and P. uncta are purely marine species, whereas M. elegans and M. moyebi are able to complete their lifecycles within the lagoon (De Bruin et al., 1994). The coastal and lagoon catch statistics of this study were in conformity with the reported compositions of the shrimps by Sanders et al. (2000) during 1998–1999, except for M. elegans and M. moyebi, which were in low numbers, comparatively lower in NMT catches, and completely absent in MT catches. Absence of M. elegans, M. moyebi, M. affinis and M. monoceros in MT but present in NMT could be due to either gear selectivity or the absence of these species in the Hendela fishing grounds, which is located further from the lagoon mouth than the Negombo fishing grounds.
Similarly, lower CPUE of P. uncta by one fourth in MT than in NMT, and by one fifth in Sanders et al. (2000) study, may be due to differences in gear selectivity or lower abundance at Hendala. Sanders et al. (2000) recorded two species, P. latisulcatus and P. canaliculatus, not encountered in this study; their not recorded P. maxillipedo, however, was caught in MT catches in low numbers.
Spatial-temporal dynamics of CPUE of shrimps
The observed fluctuations of the coastal shrimp catches and the peak CPUE durations in this study are consistent with the findings of De Bruin et al. (1994), Jayawardane et al. (2004), and De Croos and Valtýsson (2007). The peak fishing season coincides with the southwest monsoonal period in Sri Lanka. Seasonality in shrimp catches has also been reported from Jaffna lagoon in northern Sri Lanka (Chitravadivelu, 1990), India (Rajendran and Kathiresan, 1999; Iwasaki and Shaw, 2008) and many other parts of the world (Fischer and Bianchi, 1984; Potter et al., 1986), although the peak production periods are different.
The reason for the observed seasonality of CPUE values in the lagoon and coastal gear can be explained by a mass migration of shrimps towards the spawning grounds in coastal waters from the lagoon with the onset of the southwest monsoon (May–Sept.). Monsoonal rainfall lowers the salinity and may cause osmotic stress and further affect other climatic and physiochemical factors such as water level, currents, and temperature fluctuations (Garcia and Le Reste, 1981; Luchmann et al., 2008). This results in high shrimp production and relative abundance in coastal trawling gear in May–September, while lagoon production is high from January to May. In the Negombo trawling grounds, high CPUE values were observed in the southwest monsoonal period, with a peak period in January; whether this is related to the processes in the lagoon is unknown.
The trawling gear NMT and MT had considerably higher CPUE values with respect to other gear types. The percentage mean annual contributions of shrimps estimated for MT (66%) and NMT (63%) were comparable to the values reported by Jayawardane et al. (2004) in 1998–1999. According to De Croos and Valtýsson (2007), the mean proportion of shrimp contributions from the same waters for 1997–2004 was 55% (NMT) and 44% (MT); however, in many parts of the world, bycatch dominates the shrimp trawl catches (Kennelly, 2007). Metapenaeus dobsoni and P. coromondalica had the highest contributions to the total trawl catches; a similar observation was reported in previous studies (Sanders et al., 2000; Jayawardane et al., 2004), moreover contribution percentages of these two species have remained in a similar range in all studies. However, in comparison to Jayawardane et al. (2004), percentage contributions of F. indicus and P. semisulcatus to the total trawl catches were reduced from 5 to 1%. The mean annual shrimps CPUE of MT and NMT (13 and 12 kg landing−1) was slightly higher than the values reported (11–10 kg landing−1) by Jayawardane et al. (2004) but the fishing effort, in terms of number of boats operated, was reduced by 8–10% in both MT and NMT. Although the average CPUE is slightly higher in this study compared to that of 1998–1999 (Sanders et al., 2000), the total trawl catch (MT and NMT) has been reduced by 2 t. This could be due to a lower number of gear units operated and less competition, or to lower species abundance.
Shrimps contributed 70% to the total catch of SN, lower than the values (~82%) reported in 1997 and 1999 (Amarasinghe et al., 1997; Jayawardane and Perera, 2003). Although M. dobsoni contributed the highest percentage to the total catches of SN, the composition fluctuated from 57% in the earliest study by Amarasinghe et al. (1997) to 22% (Jayawardane and Perera, 2003), and to 38% in the present study. The contribution of F. indicus with 19% in the present study is higher than the 14% reported by Jayawardane and Perera (2003). High catch rates of M. dobsoni are partially due to the high number of individuals. Moreover, Amarasinghe et al. (1997) reported that M. dobsoni attained adult size before migrating back to the sea for spawning. Although mean annual CPUE was in a similar range, the total shrimp production was lower, with 20 t of shrimps and 40 t of fish than the reports of Jayawardane and Perera (2003).
Estimated total production caught by trammel nets (TN1 + TN5) was half the value (304 t) reported by Sanders et al. (2000); the CPUE was lower in shrimps by 0.4 kg landing−1 but higher in fish by 3 kg landing−1. Use of the TN has spread widely within the past decade in the lagoon and may have caused the observed reduction in mean annual CPUE, and might be an indication of reduced abundance due to higher exploitation. Similarly, the mean annual shrimp CPUE (in LM + LP + TS) is less (2.5 kg landing−1) for the CN catches compared to Sanders et al. (2000) (3.7 kg landing−1). Although there are no previous records on the total production of CN and number of gear units operating in the lagoon, it is believed that most of the cast net fishers have shifted to trammel nets in the past decade, but TN production does not appear to have increased. The highest CPUE of CN and TN observed at LP could be the high abundance of shrimps due to their habitat preference. Although habitat preference of shrimps with respect to vegetation, sediment characters, salinity and dissolved oxygen has been studied in many waters (Henderson et al., 2006; Fransozo et al., 2009) such information for the study site is scanty. Decreased numbers of FN and GN gear units in past few decades (personal communication from fishermen) might have caused their slightly higher CPUE (LP + TS) compared to Sanders et al. (2000) due to reduced competition. The mean annual shrimp compositions in BP catches are much higher (18%) than the values (5%) reported by Sanders et al. (2000) for 1998–1999. However, for the same years, Amarasinghe et al. (2002) reported a 91% fish catch while the remainder was from both Penaeid shrimps and crabs.
Comparisons of spatial-temporal differences in CPUE are justifiable, as no modifications of NMT, MT and SN gear, which have led to changes in fishing effort with time, have been recorded. Also most of these fisheries have been operating for more than 100 years (Amarasinghe et al., 2002; Gunawardena and Steele, 2008) and good knowledge of the fishing location, fish behaviour and gear operations can thus be assumed. Such a comparison for BP is problematic, as gear and effort standardisation require assumptions on the fishing method and operation, which are difficult with the available data. In Negombo lagoon, the use of light sources (lamps, fire torches, etc.) as devices to attract shrimps in SN and in some FN and CN operations, the composition and density of mangrove branches used in BP constructions, exact area of BP, and the magnitude of the much needed gear operational skills especially for CN operations are really hard to measure and standardise but could definitely have an effect on the gear selectivity and efficiency. A lack of reliable direct data from fisheries for estimating actual fishing effort and catch (including discards and bycatch) is a common challenge among all fisheries in the world (McCluskey and Lewison, 2008; Anticamara et al., 2011). This challenge can be overcome by collecting data on detailed gear information, time spent for fishing and searching, catch rates and special and temporal information across the entire fishery (McCluskey and Lewison, 2008) by the use of on-board electronic logbooks (Gallaway et al., 2003; Cole et al., 2006). However, such fully controlled surveys are expensive and may not be feasible for small-scale fisheries; thus a combination of direct observations (Lynch, 2006), interview data taken at the landing site (accurate memory) and logbooks data provide reasonable estimates of catch and effort (Cotter and Pilling, 2007; Bishop et al., 2008). But even with a strong sample design the quantification of variables may confound effort estimates, such as skipper skill and information shared among skippers, repetitive fishing in the same area, and technological advances (Hilborn, 1985; Fonteneau et al., 2004; Eigaard et al., 2011). Further, accurate catch estimates are challenging especially in small scale fisheries where illegal, unregulated and unreported (IUU) fishing are abundant (Mills et al., 2011). Walters (2003) and Maunder et al. (2006) have also shown that the standardised CPUE are not always representing the abundance. Nevertheless, comparisons of this study are pervasive as they represent the resource utilisation and are thus important for understanding the harvested resource and its management (cf. McClanahan et al., 2010).
The observed higher annual CPUE for some gear other than in the Sanders et al. (2000) findings may be due to reduced competition among gear as a result of the reduced number of gear units. According to Hilborn and Walters (1992) and Harley et al. (2001), CPUE calculated from commercial data might not be proportional to the abundance due to behaviour and experience of fishermen, but Bellido et al. (2001) suggested that CPUE could be used as an index of abundance. At any rate, comparison of 2–3 years of data limits the conclusions on exploitation patterns, as the observed CPUE changes could also be due to an annual fluctuation of environmental factors that may influence recruitment, survival and/or growth of the species (Miyahara et al., 2005).
The estimated 200 t less shrimp than during 1998/9 from the same waters (Sanders et al., 2000) could be an indicator of population decline due to increased fishing pressure by the higher number of TN usage together with habitat alteration. Moreover, comparison with the studies of Sanders et al. (2000) and Jayawardane et al. (2004), shows a reduced number of P. uncta individuals and complete absence of P. latisulcatus and P. canaliculatus in this study despite 2–3 times larger samples. Further, relatively smaller length at maturity (L50), in P. coromandelica (De Croos and Pálsson, 2011) and M. dobsoni (De Croos et al., 2011) at Hendala than at Negombo, as well as lesser mtDNA diversity in P. coromandelica (De Croos and Pálsson, 2011), F. indicus (De Croos and Pálsson, 2010), M. dobsoni and P. semisulcatus (unpublished) may reflect intensive/selective fishing at Hendala. Lagoons and estuaries in developing countries are, in general, heavily exploited by artisanal fisheries (Lae et al., 2004), as lagoons are an important source of fish to the local people and fisheries an important component of their economy (Blaber, 2000). Although commercial fisheries are not stabilised in the Negombo lagoon area, most artisanal fisheries operating large numbers of gear units may be responsible for an increased fishing effort due to an open access nature, which could lead to overexploitation as seen in many parts of the world (Blaber, 2000; Arellanop-Torres et al., 2006). It should be noted that the open access nature has been restricted by a community based management system in Negombo lagoon for SN (Amarasinghe et al., 1997; Gunawardena and Steele, 2008), and BP (Amarasinghe et al., 2002) fisheries. Overall, the fishery is showing some signs of the high fishing pressure on shrimp resources.
The selectivity of gear is associated with the length of the shrimp. The SNs are operated at the lagoon mouth, targeting migratory shrimp as revealed by the length-frequency distributions in this study. Trawl gear select larger, mature shrimp that have migrated to the coast for spawning.
There were differences in gear catch compositions. The trawl gear showed a high dominance of marine species and lower diversity in its catches. Comparatively high variability in monthly catch compositions of shrimps (Fig. 7e) might be due to their migratory behaviour between the lagoon and coast at different months of the year, but in all species analyses (7b) such dominance in catch compositions is not prominent. The diversity indexes calculated for all species within the lagoon are in a similar range for all gear except FN, perhaps due to the knowledge accumulated over the past 100–250 years for some gear (Gunawardena and Steele, 2008). Fishermen seem to be aware of the natural pattern of fish availability and operate their gear to optimise the yield (Amarasinghe et al., 2002). Thus, despite the structural, operational and spatial differences, all lagoon gear seem to be targeting all available resources, resulting in a similar range of catch compositions as seen in Fig 7b. Although the catch compositions and size frequencies are determined partly by the gear, as described by Wright and Richards (1985) and Gobert (1994), gear selectivity needs to be studied in combination with traditional fishing (see also Ruddle, 1996). Moreover, the shrimp migratory pattern along the lagoon, the proportion of mixing of species at different fishing grounds (especially at Hendala and Negombo) and the existence of any stock structures will be important in developing a sound management plan. Gear-based management may be one of the best alternatives in managing multi-gear, multi species fishery (McClanahan and Mangi, 2004; McClanahan and Cinner, 2008; Davies et al., 2009) as it is less likely to threaten fishers' livelihoods (Cinner et al., 2009). This requires baseline information on selectivity of resources and catch compositions of gear used in fisheries. Thus, the identification of fishing gear that preferentially target species is of great management importance (Cinner et al., 2009) to utilise most of the available species in a sustainable manner (McClanahan and Mangi, 2004). The information presented in this study can contribute to the development of fisheries management decisions, allowing resources to be captured without promoting resource and size overlap, in addition to improving rationalisation of future sampling protocols.