5.2 NON-RETAINED SPECIES
5.2.1 Captured in nets
5.2.1.5 Discarded fish
Syngnathids are the collective group that contain organisms such as seahorses, sea dragons and pipefish.
Syngnathids are incidentally caught in the SBP fishery and are generally removed from the codend dead. Syngnathids are a protected species under the EPBC Act.
ERA Risk Rating: Impact on breeding population (C1 L2 LOW)
The potential consequence of the prawn trawling operations on breeding levels of syngnathids was considered ‘minor’. Anecdotal evidence from observer program results has suggested that very low numbers of syngnathids are caught by this fishery, in the order of 1 per night across the whole fleet.
Furthermore, it is suggested that the occurrence of syngnathids appears to be area specific and often syngnathids may not be caught for many nights in a row. As a result, this number (1 syngnathids per night across the fleet) is more indicative of an average for the season. It was considered ‘unlikely’ that this level of consequence would result because trawling occurs over areas that are mostly unfavourable to syngnathids. Syngnathids are known to favour seagrass and detached algae communities, which are not normally trawled over.
Opportunistic data will be collected on the catch of syngnathids, by observers and other technical staff on the vessels from time-to-time. This data will continually be compiled to determine a better profile of syngnathid catches in this fishery.
5.2.1.5 Discarded fish Rationale for Inclusion
Trawling contributes to the mortality of several non-commercial fish species that are incidentally caught and die due to the disturbance they have experienced in the trawl net or from being out of water during the sorting process. These fish are discarded overboard, usually dead. In terms of volume, the sum of bycatch is between 4 and 8 times the annual prawn catch for this fishery, and small species of fish (which generally die and are returned overboard) make up the majority (at least approximately 70-80%) of the bycatch (Dr. S. Newman*, pers. comm.). The impact of this level of mortality on the sustainability of those species is explored here.
ERA Risk Rating: Impact on breeding population (C2 L3 MODERATE)
During the risk assessment workshop, the risk to the collective group of discarded fish species was considered. Using this method, the consequence of an impact to breeding populations of bycatch
species, by the activity of trawling was considered ‘moderate’. This was due to the fact that the amount of discards is large, possibly indicating that significant mortality of bycatch species occurs, however given the relatively small area of fishing compared to the overall area where these species probably occur, the mortality is probably sustainable.
In summary, of the small fish that are caught and discarded, very few are subject to other fishing mortality; therefore the trawl fishery is the only known direct activity impacting the species. Young fish, which are generally caught by trawlers, commonly have high mortality rates and as such the fishing mortality may have little additive impact on this rate. Secondly, as the species are generally only taken by trawling, maintaining a trawl impact of less than 40% on the stock distribution can be expected to keep all of the individual stocks above the maximum reference point of 0.4 of virgin biomass for most finfish species.
As trawling has occurred for at least 8 months of the year, every year, for a period of over 40 years, it is considered ‘possible’ that this level of consequence could result.
Following the risk assessment workshop, the Department of Fisheries undertook an assessment of the risk to individual discarded fish species taken in the SBP fishery. This was undertaken according to the same criteria developed and applied by Stobutzki et al. (2000), in the Northern Prawn Fishery.
Only two species of the 21 most commonly caught species (the spiny headed flounder Engyprosopon grandisquamum, and the heart headed flathead Sorsogona tuberculata), rated highly susceptible to trawling using the criteria. However, these species have high turnover rates and therefore the risk associated with fish bycatch was still rated as minor. They are also presumed to occur in areas outside of the trawled area.
The introduction of bycatch reduction devices such as square mesh panels in the net over the coming three years will reduce the overall amount of small fish caught and will substantially reduce the catch of some species.
Operational Objective
To ensure that adequate refuge areas are provided within Shark Bay, for discarded bycatch species of the SBP fishery.
Justification
Regardless of the level of impact on discarded fish species within the trawl grounds, if an adequate proportion of the populations of these species is located outside the trawl area, than this should ensure their sustainability.
Indicator
Information on the distribution of bycatch species (in particular the two more vulnerable species) both within and outside the area of trawling from a research survey of the region will indicate the proportion of the region that provides a refuge to these species from trawling.
Performance Measure
The majority of bycatch species are found in relatively significant numbers outside the trawled areas.
Justification
The Department of Fisheries will undertake a survey of species within and outside the trawl grounds to determine the proportion of refuge for bycatch species within the next five years.
Data Requirements for Indicator
Data Required Availability
Detailed daily commercial log books provide information on fishing locations by latitude and longitude for each trawl shot, which can be used in GIS analysis
Yes; available since 1998
VMS data. Each boat can be polled for location at regular intervals according to compliance protocols and this information can be utilised in spatial analysis
Yes; available since 2000
Distribution of bycatch species within the Shark Bay Region
To be determined through bycatch distribution survey, which will be undertaken within the next five years
Evaluation
An evaluation of this issue will be provided following the completion of the survey.
Robustness
Higha) Compliance policing including the use of VMS, which can validate the voluntary log book records on location and speed of vessels in the fishery and gear checks.
b) Scientifically assessed methodology for undertaking the survey in collaboration with the Western Australian Museum.
It should be noted that the interpretation of the survey data is critical due to the large degree of background variation in the natural systems, which will be investigated.
Fisheries Management Response
Current: To ensure maintenance of the required level of breeding stocks of non-target species:
a) The fishery is managed through input controls that limit boats, gear, seasons and locations of trawling.
b) The annual fishing season is for a fixed period and includes seasonal, area and moon closures limiting total effort on all species.
c) Compliance policing includes use of VMS and gear checks.
d) Development of a Bycatch Action Plan.
e) Introduction of bycatch reduction devices.
exclusion devices over the next two years, will reduce the overall bycatch taken, particularly small fish species. Preliminary trials indicate the potential to reduce some species by around 50%. From the specific experiments undertaken in Shark Bay in August 2000, there were insufficient numbers of spiny headed flounder caught for statistical analysis. For the small-toothed flounder (Pseudorhombus jenynsii), another species of flounder there was a significant reduction in the numbers (50% reduction) in the net with a grid and square mesh panel compared to the net without any bycatch reduction devices.
There was a 35% reduction in the number of heart-headed flathead in the net with grid and square mesh panel but this was not of significance due to the high variability in the numbers caught between trawl shots.
Trawl studies focussing on the biodiversity of trawled and untrawled areas in the Shark Bay Region will provide additional information on the distribution of fish species, particularly for those species that appear to be more susceptible to trawling.
Total swept area calculations are currently made each season using the overall number of hours trawled, average headrope length and spread of the gear. The use of GIS systems for analysing data has commenced and annual monitoring of the spatial extent and swept area can be made.
Actions if Performance Limit is Exceeded: The strategies that are available to offer further protection to bycatch, if required, include expanding the system of area closures or further modifications to trawl gear to reduce bycatch levels.
Comments and Action
Even though a relatively low risk was associated with this issue since no species in the assessment were ever concluded to be of more than a minor risk, the Department has developed a proposal for FRDC funding to complete the survey during the next five years.
External Driver Check List
Environmental factors such as: climatic changes, ocean currents and sea-surface temperatures are known to affect the levels of recruitment of all fish species, including bycatch species.