5.2 NON-RETAINED SPECIES
5.2.2 Interaction but no capture
5.2.2.1. protected.species.–.Dugongs.and.cetaceans Rationale for inclusion
Dugongs and cetaceans do occur in Exmouth Gulf but abundances are generally not as high as in Shark Bay. Dugongs are a protected species under Commonwealth and State legislation and any impacts as a result of this Fishery are to be managed and reduced.
ERA Risk Rating: Impact on breeding populations (C1 L3 LOW)
The interactive impact (as opposed to impact of capture) that prawn trawling activities have on the breeding populations of dugongs and cetaceans in Exmouth Gulf are considered negligible.
This conclusion is based on the operational aspects of the trawling activities and the behavioural characteristics of dugongs and cetaceans.
Firstly, trawlers do not operate at speeds greater than around four (4) knots. At such relatively low speeds it is highly unlikely that any cetacean would come in direct contact with a trawler or the gear being towed because they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
ESD Report Series No. 1 – Exmouth Gulf Prawn Fishery
Specifically, in the case of dugongs there is anecdotal evidence to suggest that the trawl grounds in Exmouth Gulf and dugong habitat (which generally refers to the seagrass habitats permanently closed to trawling activities) are spatially separated. Similarly, all of the cetaceans found in Exmouth Gulf have a distribution range far greater than that of Exmouth Gulf therefore any potentially detrimental impacts are likely to be localised and not widespread.
And finally, while prawn trawlers have operated in Exmouth Gulf for over four decades the anecdotal evidence suggests that populations of dugongs and cetaceans have not changed significantly over that time.
5.2.2.2. protected.species.–.Leatherback.turtles Rationale for inclusion
Leatherback turtles are considered a protected species under Commonwealth legislation and are found in Exmouth Gulf.
ERA Risk Rating: Impact on breeding population (C0 L5 NEgLIgIbLE)
In terms of the impact of the EGP fishery on the leatherback turtle breeding population by way of interaction (as opposed to capture), the risk assessment determined that it was ‘likely’ that the Fishery would have a ‘negligible’ impact.
Exmouth Gulf is approaching the southern end of the distributional range of the leatherback turtle.
Leatherback turtles are quite uncommon in Exmouth Gulf. There are no known breeding locations in Exmouth Gulf and it is assumed that nesting occurs in the Indonesian Archipelago.
Leatherback turtles feed primarily on jellyfish therefore they are likely to only occur in Exmouth Gulf following a jellyfish aggregation.
Regardless, at such relatively low vessel speeds (around 4 knots) it is highly unlikely that any leatherback turtle on the trawl ground would come in direct contact with a trawler or the gear being towed because they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
5.2.2.3. threatened/listed.species.–.Loggerhead.turtles Rationale for inclusion
Loggerhead turtles are considered an endangered species under Commonwealth legislation and are an inhabitant of Exmouth Gulf waters.
ERA Risk Rating: Impact on breeding populations (C1 L4 LOW)
In terms of the impact of the EGP fishery on the loggerhead turtle breeding population by way of interaction (as opposed to capture), the risk assessment determined that it was ‘likely’ that the Fishery would have a ‘negligible’ impact.
Loggerhead turtles are relatively less common than green turtles in Exmouth Gulf but have a wide distributional range. Loggerheads are generally found within shallow waters outside of the trawl grounds however they also forage over open substrate such as the mud/shell substrate that dominates the trawl grounds in Exmouth Gulf. Consequently, trawlers and green turtles may interact.
However, at such relatively low vessel speeds (around 4 knots) it is highly unlikely that any loggerhead turtle on the trawl ground would come in direct contact with a trawler or the gear being towed because
they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
5.2.2.4. threatened/listed.species.–.Green.turtles Rationale for inclusion
Green turtles are considered a vulnerable species under Commonwealth legislation and are an inhabitant of Exmouth Gulf waters.
ERA Risk Rating: Impact on breeding population (C0 L5 NEgLIgIbLE)
In terms of the impact of the EGP fishery on the green turtle breeding population by way of interaction (as opposed to capture), the risk assessment determined that it was ‘likely’ that the Fishery would have a ‘negligible’ impact. This is based on the knowledge that green turtles are by far the most abundant turtles in Exmouth Gulf and adult green turtles are herbivorous and therefore likely to forage in the shallow seagrass and macroalgal beds that are predominantly closed to trawling. Consequently, trawlers and green turtles are unlikely to interact.
Furthermore, at such relatively low vessel speeds (around 4 knots) it is highly unlikely that any green turtle traversing the trawl ground would come in direct contact with a trawler or the gear being towed because they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
5.2.2.5. threatened/listed.species.–.Flatback.turtles Rationale for inclusion
Flatback turtles are considered a vulnerable species under Commonwealth legislation and are found in Exmouth Gulf.
ERA Risk Rating: Impact on breeding population (C0 L5 NEgLIgIbLE)
In terms of the impact of the EGP fishery on the flatback turtle breeding population by way of interaction (as opposed to capture), the risk assessment determined that it was ‘likely’ that the Fishery would have a ‘negligible’ impact.
Given the preferred diet and foraging behaviour of flatback turtles it is likely that they may occur on the trawl grounds.
However, at such relatively low vessel speeds (around 4 knots) it is highly unlikely that any flatback turtle on the trawl ground would come in direct contact with a trawler or the gear being towed because they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
5.2.2.6. threatened/listed.species.–.Hawksbill.turtles Rationale for inclusion
Hawksbill turtles are considered a vulnerable species under Commonwealth legislation and are found in Exmouth Gulf.
ERA Risk Rating: Impact on breeding population (C0 L5 NEgLIgIbLE)
In terms of the impact of the EGP fishery on the hawksbill turtle breeding population by way of
ESD Report Series No. 1 – Exmouth Gulf Prawn Fishery
interaction (as opposed to capture), the risk assessment determined that it was ‘likely’ that the Fishery would have a ‘negligible’ impact.
Exmouth Gulf is well within the distributional range of the hawksbill turtle however they are relatively uncommon. The hawksbill turtle prefers foraging over hard coral and rock substrate so it is unlikely to physically occur on the trawl grounds. The coralline and rock substrates are generally outside of the trawlable areas – however, trawlers actively avoid these areas anyway given the potential damage that can occur to the gear by hard bottoms.
Regardless, at such relatively low vessel speeds (around 4 knots) it is highly unlikely that any hawksbill turtle on the trawl ground would come in direct contact with a trawler or the gear being towed because they would be able to actively remove themselves from the path. Direct physical interactions are more likely to occur with fast moving recreational craft.
5.3 GENERAL ENVIRONMENT
Taking Everything
Taking Byproduct
Taking Prawns Fishing (trophic Interactions)
Mud/shell Macroalgae Coral & Sponge Rock
Benthic Biota (direct impacts of trawling) removal of/damage to
organisms
Stock enhancement HAS BEEN TRIALED
Discarding/Provisioning
Translocation addition/movement of biological material Impacts on the biological community
through
Fuel usage/Exhaust
Greenhouse gas emissions Air quality
Debris
Oil discharge
Turbidity (eg. on aquaculture) Water quality
Substrate quality (not for this fishery) Broader Environment General Ecosystem
Figure 20. Component tree for the general environment.
Black boxes indicate that the issue was considered high enough risk at the June 2001 Risk Assessment workshop to warrant having a full reports on performance. Grey boxes indicate the issue was rated as a low risk and no specific management is required – only justification is presented.