Blue Morwong (2020)
Date Published: June 2021
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The biological stock structure of Blue Morwong is unknown, although the its lengthy pelagic larval phase suggests considerable potential for individuals from geographically separated areas to mix. Assessments are presented here at the jurisdictional level. Stocks are classified as sustainable in WA, negligible in NSW and SA, and undefined with limited data in TAS and by the Commonwealth.
Photo credit: Chris Dowling.
Stock Status Overview
|Western Australia||Western Australia||Sustainable||
Catch, catch distribution, catch rates, biology, length and age composition, fishing mortality, index of spawning stock biomass.
Blue Morwong's natural distribution is throughout the southern coastal waters of Australia's mainland. The stock structure is largely unknown. Its family, the Cheilodactylidae, typically have a pelagic larval phase lasting several months, facilitating transport over substantial distances. For Western Australia Coulson et al.  suggested that as juveniles grow, substantial numbers move from the south coast to the lower west coast where they soon mature and spawn. Larvae are then transported south and then eastwards to the south coast, which is a juvenile nursery. Here assessment of stock status is presented at the jurisdictional level—Commonwealth, New South Wales, South Australia, Tasmania and Western Australia.
In Western Australia, Blue Morwong are taken mainly by the commercial demersal gillnet sector off the lower west and south coasts. The size selectivity of the net, coupled with the larger size reached by males, results in a higher level of fishing mortality for males. Catch-at-age sampling of 2,621 south coast Blue Morwong from the demersal gillnet and recreational sectors from 2012 to 2014 suggested regular and consistent recruitment for the previous two decades [Norriss et al. 2016]. The demersal gillnet sample (n = 1,234) from the eastern sub-region of the south coast was considered the most representative for an age based stock assessment. Two alternative methods were used to generate median estimates of female spawning potential ratio (SPR, the reproductive potential of the stock at the current level of fishing mortality compared to that at an unfished level, ± 95 per cent CI): SPR1 = 0.58 (0.46–0.71) and SPR2 = 0.54 (0.41–0.68), with an almost zero chance of breaching the threshold reference point (SPR=0.30) for either method. There was a 7 per cent and 25 per cent chance, respectively, of breaching the threshold reference point (SPR=0.30). For males, SPR1 = 0.36 (0.25–0.51) and SPR2 = 0.34 (0.23–0.50) with a 19% and 31% chance of breaching the threshold reference point, respectively. There was an almost zero chance of males beaching the limit reference point (SPR=0.20). Estimates of natural mortality M (i.e. all sources of mortality other than fishing) were 0.22 (0.18–0.26) and fishing mortality F year-1 were Ffemales = 0.106 (0.072–0.137) and Fmales = 0.180 (0.123–0.231), giving a point estimates of F/M of 0.49 and 0.84 for females and males respectively. The probability of F breaching the threshold level (F/M = 1) was almost zero for females and 25% for males, and almost zero for either sex breaching the limit (F/M = 1.5).
The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired. The above evidence also indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
On the basis of the evidence provided above, the Western Australian jurisdictional stock is classified as a sustainable stock.
Blue Morwong from the south of Western Australia reach a maximum age of 24 years, are gonochorists (do not functionally change sex) with onset of sexual maturity at age 3 to 8 years at about 40 to 60 cm FL for females and 50 to 65 cm FL for males, with moderately fast growth (at age 5 years average total length was 55 cm for females and 58 cm for males) [Coulson et al. 2010, Norriss et al. 2016].
|Species||Longevity / Maximum Size||Maturity (50 per cent)|
24 years, 98 cm total length
Females: 3–8 years, 400–600 mm FL. Males 3–7 years, 500–650 mm FL.
Distribution of reported commercial catch of Blue Morwong.
|Hand Line, Hand Reel or Powered Reels|
|Rod and reel|
|Hook and Line|
|Rod and reel|
|Marine park closures|
|Effort limits (individual transferable effort)|
|Marine park closures|
|Bag and possession limits|
|Licence (Recreational Fishing from Boat License)|
|Recreational||14 t (2017/18)|
New South Wales – Indigenous (Management Methods) https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing
New South Wales – Recreational (Catch) Murphy et al. .
Tasmania – Commercial (catch) Catches reported for the Tasmanian Scalefish Fishery are for the period 1 July to 30 June the following year. The most recent assessment available is for 2018/19.
Tasmania – Recreational (management methods) A recreational licence is required for fishers using dropline or longline gear, along with nets, such as gillnet or beach seine. A minimum size limit of 250 mm is in place for all Morwong species other than Banded Morwong in Tasmanian waters. A bag limit of 10 fish and a possession limit of 20 fish (all Morwong species other than Banded Morwong) are also in place.
Tasmania – Indigenous (management methods) In Tasmania, Indigenous persons engaged in traditional fishing activities in marine waters are exempt from holding recreational fishing licences, but must comply with all other fisheries rules as if they were licensed. For details, see the policy document "Recognition of Aboriginal Fishing Activities” (https://dpipwe.tas.gov.au/Documents/Policy%20for%20Aboriginal%20tags%20and%20alloting%20an%20UIC.pdf).
Commercial catch of Blue Morwong - note confidential catch not shown.
- Coulson, P.G., Hesp, S.A., Hall, N.G. and Potter, I.C. (2010). Life cycle characteristics of the Blue Morwong Nemadactylus valenciennesi, compared with those of other species of Cheilodactylidae. Marine and Freshwater Research, 61: 104-118.
- Murphy, J.J., Ochwada-Doyle, F.A., West, L.D., Stark, K.E. and Hughes, J.M., 2020. The NSW Recreational Fisheries Monitoring Program - survey of recreational fishing, 2017/18. NSW DPI - Fisheries Final Report Series No. 158.
- Norriss JV, Fisher EA, Hesp SA, Jackson G, Coulson PG, Leary T, and Thomson AW. 2016. Status of inshore demersal scalefish stocks on the south coast of Western Australia. NRM Project 12034 Final Report. Fisheries Research Report, No. 276. Department of Fisheries, Western Australia, 116 pp.
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