RAP newsletter

Summary of findings from recreational logbook data

Across the three fisheries, 117 dedicated fishers submitted over 1,000 log sheets between June 2013 and December 2021, reporting a total catch of 25,604 blue swimmer crabs. The greatest logbook activity was recorded in the SCE, with 239 trips and 3,026 crabs reported during 2015/16 alone. While fewer trips were reported from the LE and GB, crab catches were still generally high in these areas (Figure 1).

Drop netting was by far the most popular crabbing method, accounting for nine out of every 10 crabbing trips. Drop netters used over 20 different bait types, with the most popular being fish (heads, frames), spleen and chicken (frames, necks and wings). Scoop netting was the next most popular method in the LE and GB, while diving or snorkelling was popular in the SCE and resulted in the capture of large sized crabs (average 153 mm carapace width [CW]).

The most popular months for crabbing in the SCE and LE were December to February, accounting for 60–70% of total annual catch and effort (Figure 2). Catch and effort In GB tended to be more evenly spread from August through to January each year.

Release rates of crabs varied between regions and years. In the SCE, 19-47% of captured crabs were released, while in LE and GB as many as 60–86% of crabs were released in most years (Figure 1). The majority of released crabs were undersize or carrying clutches of eggs, but on occasion fishers also released legal sized crabs. The main reasons for releasing legal sized crabs were preferences for larger-sized crabs and to return all female crabs to protect the breeding stock.

The size of crabs caught ranged from 40 to 210 mm CW. Good sized males between 127 and 167 mm CW were consistently caught each year in the SCE (Figure 3). In the LE a much higher proportion of catches were below legal size, with many young males ranging 76–127 mm CW and a maximum size of 163 mm. In contrast, catches in GB were dominated by female crabs ranging 97–147 mm, with the largest being 192 mm. A reasonably high proportion of large berried females (127–147mm) were also recorded between October and December in GB.

Between 2013/14 and 2018/19 the annual drop net catch rate for reporting crabbers in the SCE ranged from 1.1–1.7 kept crabs per 10 drop net lifts, increasing significantly to 3.0 kept crabs per 10 drop net lifts by 2020/21 (Figure 4). This compares with the long term (8-year) SCE average of 1.6 kept crabs per 10 drop net lifts. Catch rates in the LE between 2013/14 and 2020/21 ranged from 0.7–3.8 kept crabs per 10 lifts (average of 2.3), and from 1.4–2.5 kept crabs per 10 pulls (average of 1.9) in GB over the same period.

Figure 1 (above) - Annual catch and effort statistics for SWRCP logbook fishers reporting crabbing trips in the Swan-Canning Estuary, Leschenault Estuary and wider Bunbury area and Geographe Bay between June 2013 and December 2021. Each fishing year is based on a 12-month season spanning June 1^st to May 31^st the following year.

Figure 2 (above) - The mean monthly proportion of crabs caught, and crabbing trips undertaken, by SWRCP logbook fishers between June 2013 and December 2021.

Figure 3 (above) - Annual length frequency distributions of male and female blue swimmer crabs caught by SWRCP fishers in the
Swan-Canning Estuary (SCE), Leschenault Estuary and wider Bunbury area (LE) and Geographe Bay (GB) for the last four seasons (June 2017 to May 2021). The dashed line represents the recreational minimum legal size of 127mm carapace width.

Figure 4 (left) - Annual average drop net catch rates (± 95% confidence intervals) of SWRCP fishers in the Swan-Canning Estuary (SCE), Leschenault Estuary and wider Bunbury area (LE) and Geographe Bay (GB) between June 2013 and May 2021. The dashed lines represent the eight-year average catch rate for each fishery from 2013/14 to 2020/21.

While annual egg count estimates in the SCE were consistent between 2103 and 2016 (7-13 million eggs per research traplift), the annual egg count fell to 4 million eggs per traplift in 2017 and just 1.6 million eggs per traplift in 2018 (Figure 6). Unseasonably heavy rainfall in February 2017 (137mm total rainfall, the wettest February in over 60 years) caused the estuary to become unusually fresh for that time of year, leading to widespread low oxygen conditions in the deeper waters of the estuary which placed additional stress on SCE crab stocks. However, blue swimmer crabs are a short-lived, fast growing species that produce large numbers of eggs so a return to more normal environmental conditions saw the egg count back to historic levels from 2019 (Figure 6).

A similar pattern was evident in LE, with annual egg counts consistent between 2013 and 2016 (11-16 million eggs per traplift) before declining to 6 million eggs per traplift in 2017. However, a return to normal environmental conditions saw the annual egg count recover to 20 million eggs per traplift by 2020 (Figure 6).

As GB is a marine embayment, this fishery is not subject to the dramatic changes in environmental conditions flooding events can cause in estuaries like SCE and LE. Consequently, annual egg count estimates in GB have been comparatively consistent between 2013 to 2021 (ranging from 10 to 28 million eggs per trap lift).

The change in breeding stock levels in the SCE each year had a direct influence on the subsequent numbers of juvenile crabs caught in recruitment trawls six months later. Numbers of recruiting juvenile crabs declined from 16 juveniles per 100m² trawled in 2016 to just 5 juveniles per 1000m² trawled in 2017 following that February’s extreme flooding event (Figure 7). The low breeding stock levels in 2017 and 2018 resulted in low numbers of juvenile crabs in 2018 (6 juveniles per 1000m2 trawled) and 2019 (3 juveniles per 1000m2 trawled), before recovering strongly from 2020 (Figure 7).

Figure 6 (above) - The average number of eggs (× 1,000,000) recorded per trap lift (± 95 % confidence intervals) during annual breeding stock surveys in the Swan-Canning Estuary (SCE), Leschenault Estuary and wider Bunbury area (LE), and Geographe Bay (GB) between 2013 and 2021. The dashed line represents the nine-year average for each fishery.

Figure 7 (above) - The average number of juvenile crabs per 1000m2 trawled during annual recruitment stock surveys in the Swan-Canning Estuary between 2013 and 2021.