Estimating the spawning fraction of Blue Mackerel off eastern Australia: Stage 1 Developing a sampling method(s) and identifying sampling locations.

Background and Need

Blue Mackerel (Scomber australasicus) is a target species of the Commonwealth Small Pelagic Fishery (SPF). Catches from the Eastern sub-area reached 10,050 tonnes in 2021/22, which is 87% of the Total Allowable Catch (TAC) of 11,440 tonnes. Estimates of spawning biomass obtained using the Daily Egg Production Method (DEPM) are used to set TACs. Limited understanding of spawning fraction (i.e., the proportion of females that spawn each night during the spawning season) off eastern Australia is a major contributor to uncertainty in estimates of spawning biomass. In December 2020, the SPF Resource Assessment Group (RAG) identified that robust estimates of the spawning fraction of Blue Mackerel off eastern Australia were needed to provide more robust and accurate estimates of spawning biomass and ensure confidence in the setting of TACs.

Objectives

The objectives of this project are to: 

1.      Establish reliable methods for sampling large adult Blue Mackerel in offshore waters of the east coast of Australia.

2.      Identify several locations (at different latitudes) where large adult Blue Mackerel can be sampled during the spawning season.

3.      Develop preliminary estimates of the spawning fraction (and other adult reproductive parameters) of Blue Mackerel off eastern Australia. 

4.      Investigate whether, or not, large Blue Mackerel spawn off southern NSW during summer. (Additional objective added at request of industry.)

Methods

Sampling was undertaken during 2022-23 between Coffs Harbour and Eden using a variety of approaches, including catch sampling from commercial mid-water trawlers and purse-seine vessels, and targeted fishery-independent sampling from commercial line-fishing vessels. Sampling focused on the latter half of the spawning season from September-October 2022 at known spawning locations such as Coffs Harbour and Port Stephens. However, sampling was also undertaken off southern NSW from September 2022 to March 2023 to determine if Blue Mackerel also spawn in this region. A variety of line fishing techniques were trialled. Standard techniques (e.g., Ward et al. 2009) were used to macroscopically stage ovaries (Stage I-V), microscopically stage oocytes (1-5), determine the presence/absence of post-ovulatory follicles (POFs) and estimate spawning fraction.

Results, Discussion and Recommendations for Stage 2

Methods for sampling large adult Blue Mackerel

Blue Mackerel above the size at maturity were collected by mid-water trawling, purse-seining and line-fishing. However, only samples taken by purse-seining off Wollongong and line-fishing off Coffs Harbour and Port Stephens included mature fish. The most effective baits for catching large mature Blue Mackerel varied among locations and fishing trips and was best determined at each site by trial and error. However, the consistently most efficient baits were fish-strips (~4 cm x 1 cm x 1 cm).

Sampling locations and timing of spawning

Mature females were collected from Coffs Harbour, Port Stephens and Wollongong during September and October 2022. POFs indicative of recent spawning were abundant in samples collected from Port Stephens in early October (present in ~24.85% of females) but rare in samples collected from Coffs Harbour (0.87%) and Wollongong (3.26%). Results are consistent with the current understanding that the main spawning season off eastern Australia extends from July to October and the main spawning area extends from Sandy Cape to about Wollongong. No evidence was collected to suggest that Blue Mackerel spawning occurs south of Wollongong.  

Preliminary estimates of spawning fraction

Based on the assumption that POFs persist for two days, the mean spawning fraction at Port Stephens in October was 0.124, which is similar to the overall estimate for South Australia (i.e., 0.135) that was obtained using the same assumption (Ward et al. 2009). Determining how long POFs persist in the ovaries of Blue Mackerel off eastern Australia is a high priority.    

Recommendations for Stage 2

Stage 2 of this study should involve sampling multiple locations throughout the main spawning area and spawning season using both commercial purse-seine vessels and targeted line-fishing. Several additional sampling sites should be established. Conducting around the clock sampling over multiple days to determine how long POFs persist in ovaries is a high priority.