This study presents results from an experimental research that was designed to quantify the effects of Jones-Davis BRD in Cleveland Bay, Townsville, Australia. A) The ratio of bycatch and target species was 47:1; B) the biomass of prawn was 41% higher in the BRD net; C) the Jones-Davis BRD reduced total fish biomass caught in present study. The total fish biomass of reduction net was 22% less in the BRD; D) the biomass of total fish without Leiognathidae was 43% less in BRD; E) the biomass of carangids was significantly reduced by 47% in BRD net; F) however, the Jones-Davis BRD did not reduce the biomass of Leiognathidae. It was 4% higher in BRD net; G) the biomass of other fish was 51% less in BRD than Control net. The results demonstrated that the Jones-Davis BRD is not effective device to reduce the biomass of non-target fish in Cleveland Bay.
It needs more research on Jones-Davis BRD in Cleveland Bay to figure out why it is not effective.
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Commercial fishing is one of the most urgent threats to the world's remaining fish stocks, especially the indiscriminate capture of non-target organisms, typically referred to as bycatch (Pauly et al. 1998; Pauly et al. 2002; Worm et al. 2006; Kumar et al. 2006). The non-target organisms may be unusable or unwanted for a variety of regulatory and economic reasons and subsequently thrown back to sea, often dead or dying (Harrington et al. 2006; Catchpole et al. 2007). This unutilized sub-set of bycatch is known as discards. The role of bycatch in degrading marine ecosystems has made this one of the most significant nature conservation issues in the world today (Harrington et al. 2006; Lewison et al. 2004; Hall et al. 2000).
Estimating the quantity of world bycatch is difficult and highly controversial. The world bycatch estimates have been undertaken since the 1970's. The previous studies demonstrated that global bycatch estimates ranged from 3 - 21 million tonnes per year (Andrew and Pepperell 1992) and estimated that global discards were close to 6.72 million tones, approximately 12% of the total annual marine fisheries landings (Saila 1983).
Therefore, scientists have tried to reduce bycatch of non-target organisms in trawling prawn and shrimp. The most obvious method of reducing the impact of trawl fishing on non-target organisms is to reduce the quantity of bycatch landed and discarded (Stobutzki et al. 2000). So a number of devices, named Bycatch Reduction Devices (BRDs) or TED's (Turtle Excluder Devices) have been developed and implemented around the world (McGilvray 1996). In the last decade substantial research effort into BRD development has produced reductions in landed bycatch biomass (Van der Geest 2000).
Technically, Jones-Davis BRDs are classified as two types, passive and active (McGilvray 1996). Passive BRD is using a physical sorting method, such as metal grids or mesh panels, to sort the target and bycatch species based on size (McGivray 1996), and restricting larger organisms from entering the net codend. Passive BRD is also known as "hard" BRD, due to the metal construction. While, active BRD rely on the behavioural difference of the target and bycatch species, by providing possible escape routes for smaller bycatch organisms (McGivray 1996). Active BRD is also termed "soft" BRD (Figure 1).
Figure 1. The Jones-Davis BRD. a) the position of the passive or hard BRD, b) the location on the active or soft BRD.
Research experiments on bycatch reduction devices (BRDs) have shown many types to be effective at reducing the bycatch of fishes in shrimp and prawn trawls (Isaken et al. 1992; Broadhurst 2000; Watson et al. 1999). However, few have been quantitatively evaluated after adoption to determine what level of bycatch reduction was achieved in the fishery. In addition, when BRDs have been evaluated in fisheries, performance has often been less than expected based on research trials (Hannah and Jones 2007). For example, in the Gulf of Mexico shrimp trawl fishery, observer data showed that Gulf Fisheye BRD performance in 1999 - 2003 was below expectations (Foster 2004). But, on the other hand, this also demonstrated the importance of fishery-scale evaluations of BRD performance. Trying to increase the effectiveness of BRDs to reduce the biomass of bycatch landed in trawl fisheries is believed to increase the long-term sustainability of the target species stock and of the fishery (Brewer et al. 1998).
Here, the present study was to examine the performance of Jones-Davis BRD in Cleveland Bay, Townsville, Australia. It was to test whether Jones-Davis BRD can reduce catch on non-target organisms in trawling and increase catch of shrimp and prawn.
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Cleveland Bay is located at Townsville, Australia (19Â°10' S and 146Â°50' E), with an area of approximately 225 square kilometers (Stark et al. 1975; Van der Geest 2000) (Figure 2). The depth range of Cleveland Bay is 0 - 15 metres and depth gradient is 1m/km until the 10 metres bathymetric mark, then increasing to 2 m/km out to 15 m depth (Carter et al. 1993; Van der Geest 2000). The epibenthos is diverse with 175 demersal fish species having been recorded in this bay (Watson et al. 1990; Yap 1990; Sondita 1997). The spatial distribution of tropical demersal fish species has very little variation in Cleveland Bay, but is attributed to depth effects (McManus 1986). In addition, the distribution of the mobile benthic community is likely to be influenced by productivity and turbidity, making the inshore areas more species rich (Stark et al. 1975). Furthermore, the Leiognathidae dominate the demersal community of Cleveland Bay, many other species are present as well (Van der Geest 2000).
Figure 2 The map of Cleveland Bay with sampling locations in inshore.
Sampling design and sample collection
The study was conducted with two types of net: one is Jones-Davis BRD; the other net was a control one, used in the commercial prawn trawl fishery. Both nets sampled at the same time approximately 15 - 25 metres apart. This survey was assumed to be sampling the same population of demersal fish independently and only sampled at the location of inshore.
All fieldwork was carried out in Cleveland Bay (Figure 2) on the RV James Kirby. The James Kirby is 19.5 metres long and has a 5.2 metre breadth. The trawl nets are standard 4.5 cm diamond mesh. Each trawl was conducted in a randomly chosen part of the location (Figure 2), with 24 locations. In addition, each trawl fished for 10 minutes, and each net sweeping approximately 6000 mÂ². All the fish were grouping when they were landed. The wet weight biomass of each grouping was measured and recorded.
To test the effectiveness of the Jones-Davis BRD the Wilcoxon test was used. The biomass data for prawn, carangids, leiognathids, other fish, total fish, total fish without leiognathids. Biomass is expressed as kilograms per 6000 mÂ². To detect the effect of BRD on the captured demersal fish community of Cleveland Bay Statistica 9 software was used in this study.
The ratio of biomass of total bycatch to target species landed in the present study was 47: 1 (Figure 3). There was much less target species biomass landed than fish in present study. In addition, the BRD net recorded a ratio of bycatch and prawns were 214.5:1, and the control net 386:1.
Figure 3 The ratio of bycatch and target species in Cleveland Bay, Australia.
The mean biomass of prawns captured in present study consistently higher in the BRD net (Figure 4). The biomass of prawns was 41% higher in the BRD net, with p= 0.65 (Table 1).
Figure 4 The prawn biomass recorded in Cleveland Bay between BRD and Control net.
The Jones-Davis BRD reduced total fish biomass caught in present study (Figure 5), with a significant reduction in BRD net (p=0.001, Wilcoxon test) (Table 1). The total fish biomass of reduction net was 22% less in the BRD. Similarly, the biomass of total fish without Leiognathidae was 43% less in BRD (Figure 6), with a significant difference between BRD and Control net (p=0.00028, Wilcoxon test).
Figure 5 The total fish biomass recorded in Cleveland Bay between BRD and Control net.
Figure 6 The total fish biomass without Leiognathids in Cleveland Bay between BRD and Control net.
The reduction in biomass of bycatch recorded from the Jones-Davis BRD was variable. The biomass of carangids was significantly reduced by 47% in BRD net (Figure 7), with p=0.00042 (Table 1). However, the Jones-Davis BRD did not reduce the biomass of Leiognathidae. It was 4% higher in BRD net (Figure 8), with a significant difference between BRD and Control net (p=0.018, Wilcoxon test).
Figure 7 The Carangids biomass recorded in Cleveland Bay between BRD and Control net.
Figure 8 The biomass of Leiognathids recorded in Cleveland Bay between BRD and Control net.
The Jones-Davis BRD reduced the biomass of other fish significantly in present study. The biomass of other fish was 51% less in BRD than Control net (Figure 9), with a p<0.001 (Table 1).
Figure 9 The other fish biomass recorded in Cleveland Bay between BRD and Control net.
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Table 1. Summary table of Wilcoxon test results from Cleveland Bay. The nets with (BRD+) and without (BRD-) the Jones-Davis BRD in trawls were treated as a pair.
The performance of Jones-Davis BRD
In present study, most of small-fish bycatch are about the same size as the commercially prawns. Consequently, excluding fish bycatch based on size selectivity is difficult. Compared to previous study, the trial carried out on the Jones-Davis BRD was testing in the Gulf of Mexico. The Jones-Davis BRD reduced the total fish biomass by 58% in the Gulf of Mexico (Watson and Foster 1997). However, in present study in Cleveland Bay the reduction in total fish biomass was only 22%. The result demonstrated that the effectiveness of the Jones-Davis BRD was affected by the differences in community composition between Gulf of Mexico and Cleveland Bay. In Gulf of Mexico, the fisheries are dominated by large, strong swimming, and pelagic species such as snapper, croaker and porgy (Van der Geest 2000). However, the demersal fish community in tropical Townsville, Australia is dominated by small, benthic species such as Leiognathidae (Van der Geest 2000). To the point of this view, Jones-Davis BRD is not effective device to reduce the biomass of non-target fish in Cleveland Bay. But, another previous study mentioned that one of the effective devices is the Super Shooter - excluded up to 16% of small fish with no significant loss of prawns. Further development of BRDs to reduce the small-fish bycatch is likely to increase these exclusion rates for prawn fishery (Brewer et al. 1998).
The temporal variation is most significant for those species whose abundance is related to the seasonal within Cleveland Bay. The biomass of the target species, especially prawns, was highly variable in the present study. The abundance of prawns should be greatest abundant at this moment (the present study was conducted in wet season in March) (Van der Geest 2000). The results (the ratio of bycatch and target species was 47.1) suggested that the device is not specific enough to reduce the bycatch biomass in the demersal fish in Cleveland Bay. Therefore, the benthic species in Cleveland Bay resulted in the Jones-Davis BRD being less effective in reducing fish biomass caught than it has been implemented in Gulf of Mexico. In other words, the Jones-Davis BRD is better suited to prawn fisheries where the benthic community composition is large species.
When it comes to the spatial variation in bycatch from a prawn fishery, the fish community composition was more affected by site location than by sampling time. Some studies have found highly significant effects due to location in prawn trawl bycatch, but no significant effects due to season, or year (Stobutzki et al. 2001; Kennelly et al. 1998). The present study was only sampling the inshore site, so to find out the reason why the Jones-Davis BRD is not effective in Cleveland Bay, further study need to do more samples in the locations of mid-shore and out-shore.
In conclusion, the Jones-Davis BRD was not effective to reduce the biomass of non-target species in Cleveland Bay. Moreover, the present study also was not catching relatively more prawns due to some reasons. So it needs more research on effectiveness of the Jones-Davis BRD in Cleveland Bay in spatial scale.