The influence of the La Niña-El Niño cycle on giant mud crab (Scylla serrata) catches in Northern Australia

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Abstract

Mud crabs (Scylla spp.) are a high value commodity harvested in the Indo-West Pacific. Scylla species support important artisanal fisheries in south-east Asia and intensive commercial fisheries in Australia where the market demand and catch has increased markedly over the last decade. Over-fishing of Scylla spp. has been observed at varying levels throughout its distribution. Fluctuations in catch rates and abundance are thought to be driven by climate parameters. Here we analyse monthly, seasonal and annual patterns in catch and effort data (from 1990 to 2008) for the commercial giant mud crab (Scylla serrata) fishery in the Northern Territory, Australia, with corresponding climatic data (rainfall, freshwater runoff, sea surface temperature) and the Southern Oscillation Index (SOI) as an indicator of La Niña/El Niño events. Between 30 and 40% of the variation in catch per unit effort can be explained by rainfall and SOI alone. This result was supported by linear mixed models which identified SOI as the main contributor to the model. Spectral analyses showed that catch peaks coincided with a four year La Niña cycle. One- and two-year time lags (consistent with S. Serrata’s life cycle) were also significantly correlated to SOI values and rainfall. These outcomes may assist fishery managers in planning fishing exposure period and duration. Furthermore, findings of this study provide information on the vulnerability of S. serrata to fluctuations in environmental conditions and can help to apply protective measures when and where necessary.

Highlights

► La Niña/El Niño events may explain 30–40% of Scylla serrata catches. ► Significant dependence of Scylla serrata on temperature and rainfall. ► Two-year lag effects of climate variables on catch rates. ► Improvements for modelling and management of Scylla serrata

Introduction

Mud crabs of the genus Scylla have a wide distribution in Indo-West-Pacific estuaries supporting important fisheries in South Africa, Pakistan, Japan, Taiwan, Philippines, Malaysia, Vietnam, China and Australia where their capture generates significant revenue for coastal communities and forms an important component of small-scale fisheries (Le Vay et al., 2001). The genus Scylla contains four recognised species: Scylla paramamosain, Scylla tranquebarica, Scylla olivacea and Scylla serrata (Keenan et al., 1998, Imai et al., 2004), with the latter two found in Australian waters. A combination of over-fishing and habitat loss has resulted in reduced landings and smaller mean size at capture in several south-east Asian countries (Le Vay et al., 2001), making Scylla spp. potentially vulnerable to environmental stressors (Hamasaki, 2003). The fisheries dynamics have rarely been modelled or assessed throughout Scylla’s wide geographic range (Overton et al., 1997). Understanding more about ecological relationships with S. serrata catches may be useful not only for managers but for understanding how this important species varies in its population characteristics and preferences for environmental conditions throughout its range.

Scylla serrata abundance appears to be strongly linked with the prevailing environmental conditions during their life history, especially during the larval and juvenile phases (Ruscoe et al., 2004). S. serrata fisheries are typically subject to high fishing mortality rates, with little carry over of stock from one year’s cohort to the next (Lebata et al., 2009). This combination of factors can result in extreme inter-annual variation in S. serrata catches. For example, the years 2000 and 2001 saw record S. serrata catches in northern Australia, presumably due to a combination of high fishing effort and favourable recruitment in the preceding years. This peak was followed by a significant decrease in catch. This phenomenon is thought to be due to one or more environmental drivers, such as rainfall/river flow or water temperature.

Freshwater flow has a significant effect on estuarine fisheries production, with the abundance and distribution of aquatic communities changing with seasonal and inter-annual variations in flow (Robins et al., 2005, Meynecke et al., 2006, Gillson et al., 2009).

The strong dependence of Scylla spp. on salinity is a world-wide phenomenon. Bonine et al. (2008) reported for Micronesia that offshore migration in tropical Scylla serrata populations is stimulated by a decrease in salinity. Similarly, seasonal changes in salinity have been reported by Walton et al. (2006) from the Philippines as an important factor in relation to recruitment of Scylla paramamosain. In India low salinity (or 2–3 ppt) reduced catches of juvenile Scylla spp. to zero, though the species is unclear (Chandrasekaran and Natarajan, 1994). Hill (1975) reported that heavy floods (resulting in salinity of 2 ppt) greatly reduced mud crab catches (suspected S. serrata) in two South African estuaries and in Australia S. serrata catch per unit effort (CPUE) was negatively correlated with salinity (24–35 ppt) but positively correlated with temperature (Williams and Hill, 1982).

Robins et al. (2005) also reported the effects of flow on Scylla serrata, and suggested that flow affects recruitment supporting the results of Loneragan and Bunn (1999) who proposed salinity change as the mechanism of adult movement. The downstream movement of mud crabs (suspected S. serrata) following floods was also reported by Stevenson and Campbell (1960). This small-scale migration of adult S. serrata may reduce the severity of cannibalism (which is a strong mortality factor in brachyuran crabs high-density environments), and burrow competition such that juveniles prosper and overall S. serrata abundance increases (Møller et al., 2008).

The difference in distributions of the four Scylla species is also suggested to be a result of varying tolerances to salinity at larval or juvenile stages (Le Vay et al., 2001). Scylla serrata in Australia is more dominant in oceans and mangroves with high salinity (about 34 ppt), and may experience higher mortalities with sudden salinity decreases associated with freshwater flooding. Conversely, the species dominant in east and south-east Asia, S. paramamosain, prefers estuarine habitats where salinities are lower than 33 ppt and maintains high catch rates through seasonal periods of low salinity and freshwater conditions (Le Vay et al., 2001).

The optimal water temperature for Scylla serrata is between 28 and 32 °C with significant lower survival at temperatures <20 °C (Heasman et al., 1985, Robertson, 1996, Ruscoe et al., 2004). The peak mating activity is usually in spring or at the end of the dry season. Therefore, spawning is often linked to the wet season (e.g. Hill, 1994) when the optimal water temperature for larvae of 28–30 °C occurs (Baylon, 2010). However, spawning can take place throughout the year depending on the region and environmental conditions.

Here we aim to use the Northern Territory (NT, Australia), Scylla serrata catches as a case study to define the most important climate drivers for S. serrata catch variability and develop models capable of enhancing the prediction of S. serrata abundance. We hypothesis that high sea surface temperature (SST) and high rainfall boost coastal productivity, and consequently, result in a positive relationship between these combined factors and S. serrata CPUE. Similarly, we expect maximum positive Southern Oscillation Index (SOI) values indicating strong monsoonal effects (La Niña phase) that result in high rainfall and warm temperatures in northern Australia, to be positively correlated with S. serrata CPUE. However, prolonged flooding (in the order of weeks to months), is expected to result in high mortality of juveniles, which in turn would have a negative impact on subsequent recruitment. Depending on the recruitment time of S. serrata to fisheries, there will be lag effect for physical environmental driver/s and the resultant impacts on catch and estimated abundance.

Section snippets

Conceptual model

Temperature and salinity have the largest influence on the life history of Scylla serrata (Hill, 1974) with the impact of environmental factors on S. serrata being greatest during larval and juvenile stages (Nurdiani and Zeng, 2007). Catch data from adult S. serrata therefore does not necessarily reflect an immediate response to temperature and salinity fluctuations. The time lag between the environmental change and effect on CPUE is expected to decrease with advanced life stages and also

Northern territory catch

Commercial Scylla serrata catch in the NT increased from 150t in 1990 to over 200t in 1994. In 1995, the catch almost tripled and reached a peak, which was followed by a slight drop in 1997. From 1998 to 2001 catches increased to over 1000t and then dropped sharply to 350t in 2002. In the following years a gradual increase to gt; 500 t was evident. We identified four periods of high catch rates consistent with the 1995/96, 1999-2001, 2004/05 and 2008/09 La Niña phases (Fig. 3). The initial

Discussion

Temporal analyses of the NT commercial Scylla serrata fishery catch revealed significant variation in catch rates. Such variations are most likely due to environmental factors affecting recruitment success over time and location throughout S. serrata geographic distribution.

In the late 1990s and early 2000s, large increases in catch and CPUE occurred in the NT. Similar patterns of fluctuation in recruitment and relative fishing effort (i.e. up to eight-fold) would have been required to explain

Conclusion

The lack of any real effort, throughout Scylla’s wide geographic range, to model the fishery dynamics of this species group, needs to be addressed in future research. Here we have focused on Scylla serrata from northern Australia but the biology and ecology of the four Scylla species seem to vary (although comparative data is next to non-existent) so the impact of environmental fluctuations may differ requiring the development of species-specific and location-specific models. Our case study

Acknowledgements

We would like to thank the Fisheries Division, NT DoR for the use of commercial Mud Crab Fishery logbook data and the NT NRETAS for supplying freshwater flow data. The Australian Government Bureau of Meteorology is also thanked for the provision of rainfall data and Peter Bayliss from CSIRO for helping with flow modeling. Jordyn de Boer is thanked for proof reading the draft manuscript and we are thankful for the reviewer’s comments. This research was funded by the Australian Government through

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