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Government of Western Australia - Department of Fisheries

Protecting pink snapper spawning in Cockburn and Warnbro Sounds

Cockburn and Warnbro Sounds support the largest known spawning schools or ‘aggregations’ of pink snapper in the West Coast Bioregion and are critical for sustaining adequate breeding stocks of this long lived species across the bioregion. The Cockburn and Warnbro Sounds annual pink snapper spawning closure, which now runs from 1 September to 31 January year since it was extended, aims to provide targeted protection for this important spawning event. The closure has been a key management measure for pink snapper sustainability in the West Coast Bioregion since 2000.

Importance of protecting spawning aggregations

Pink snapper in the West Coast Bioregion are currently in recovery. Recreational catch of pink snapper in the Metropolitan and South-West is mainly made up of relatively young fish and little or no fish over 20 years old (see Figure 1). With pink snapper living to 40 years of age, this highlights that there is still a way to go to before pink snapper stocks are considered to have recovered on the West Coast.

Figure 1. Preliminary age distribution for legal pink snapper collected from recreational fishers in the Metropolitan and South-west areas from 2015-17.

Pink snapper spawning activity and survival of larvae and juveniles is highly variable from year to year (we call this ‘variable recruitment’) with typically only one to two strong recruitments each decade. Recent good catches of pink snapper by recreational and charter fishers in the Metropolitan and South West areas have mainly comprised of a limited number of strong recruitment years. For example, fish from a strong recruitment in 2007 started showing up in the fishery as four-year-old fish in 2011 and were 10 years old in 2017 (Figure 1). This highlights the importance of protecting key pink snapper spawning aggregations to ensure pink snapper stocks are able to spawn successfully and boost the stock when favourable environmental conditions occur.

New research on spawning pink snapper in Cockburn and Warnbro Sounds

Pink snapper are usually in spawning condition from August to January in the Metropolitan area, including Cockburn and Warnbro Sounds. Spawning activity usually peaks in spring (around November) when water temperatures are about 19-21C° – the grey bars in the graph below show the typical percentage of mature/spawning female pink snapper in each month (Figure 2)

Figure 2. Average gonadosomatic index of adult female pink snapper (black line) and the typical monthly percentage of female pink snapper that are mature/spawning (bars) in the Metropolitan Area (Source: Wakefield 2010).

New research using acoustic tags shows that pink snapper use coastal reefs such as five fathom bank as pathways for migration to and from Cockburn and Warnbro Sounds. Tagged snapper only entered and left Cockburn Sound via the channel between Garden and Carnac islands. During this predictable migratory pre- and post-spawning phase, pink snapper are particularly vulnerable to fishing.

This new research also found that pink snapper being to gather in Cockburn Sound in August and September in readiness to spawn, well before the start of the current closure (Figure 3). The green dots show where tagged pink snapper were detected by acoustic receivers (squares, triangles and circles represent receiver locations) with larger green dots indicating a greater proportion of tagged pink snapper detections.

 

Figure 3. Proportions of tagged pink snapper detected at each receiver as they migrate to and from Cockburn Sound. Red shading indicates the northern part of the area closed to fishing for pink snapper from 1 October to 31 January (Source: Crisafulli et al. 2019).

Fishing pressure

Over the last few years commercial and charter catch returns and recreational fishing social media content has shown targeted fishing pressure on spawning pink snapper is occurring prior to the Cockburn and Warnbro Sounds pink snapper spawning closure. There is also evidence that recreational and charter fishers target migrating pre-spawning pink snapper on grounds adjacent to Cockburn Sound, Warnbro Sound and Owen Anchorage (e.g. Five Fathom Bank) leading up to and during the current closure.

The targeting of the spawning aggregations and migrating pre-spawning pink snapper impacts on the effectiveness of the spawning closure and therefore on the pink snapper stock as a whole. These impacts include interrupting spawning behaviour, reducing the number of fish that can spawn and disrupting spawning itself. Studies also show that capture and release of some demersal species, prior to spawning, can cause reduced spawning success or even for a fish to skip spawning in that year.

Last modified: 15/07/2019 1:47 PM

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