Exploring spatio-temporal changes in coastal recreational fisheries and potential links to extreme weather events.

Extreme weather events across coastal environments are expected to increase in frequency under predicted climate change scenarios. These events can impact coastal recreational fisheries and their supporting ecosystems by influencing the productivity of fish stocks or altering behaviours and decision-making among fishers. Using off-site telephone/diary survey data on estuarine and oceanic recreational fishing activity in eastern Australia, we analyse interannual and geographic variability in bream (Acanthopagrus spp) and snapper (Chrysophrys auratus) catch, total effort and total catch per unit effort (CPUE) through a period (2013/2014, 2017/2018 and 2019/2020) that encompassed severe drought, bushfires and flooding. Interacting spatial and temporal differences were detected for bream and may reflect spatial variation in the intensity and extent of some of the extreme weather events. The catch of snapper did not change temporally, providing little evidence that this species' catch may be influenced by the extreme weather events. Independent bioregional and temporal effects on effort were detected, while CPUE only showed significant bioregional differences. Although adverse conditions created by the extreme weather events may have dissuaded fisher participation and impacted effort, we propose that the observed temporal patterns in effort reflect the early influence of socio-economic changes brought on by the COVID-19 pandemic on coastal recreational fishing, over and above the impacts of extreme weather events. This study demonstrates how interrelated ecological, social and economic factors can shape coastal recreational fisheries and facilitates development of management strategies to address future threats to the sector.

The influence of rainfall on recruitment success and commercial catch for the large sciaenid, Argyrosomus japonicus, in eastern Australia.

Freshwater flows into estuaries influence fish populations through effects on recruitment, growth and mortality. We compared year class strength of a large sciaenid Argyrosomus japonicus with rainfall through a 16-year period in southeastern Australia, to understand the influence of freshwater input on this estuary-dependent and depleted population. Relative year class strength, estimated by back-calculating age composition data from commercial fishery landings, was positively related to rainfall within estuarine catchments (R2 = 52%). Commercial estuarine landings from a separate 29-year dataset were positively related to rainfall two and three years earlier (R2 = 21% and 44%, respectively). Year class strength of a key prey species, the penaeid prawn Metapenaeus macleayi, was also related to year class strength of A. japonicus (R2 = 39%), suggesting that rainfall influences recruitment of A. japonicus by affecting food availability. Several years of above average rainfall may be required to promote recruitment substantial enough to rebuild the population.

Reduced exploitation is associated with an altered sex ratio and larger length at maturity in southwest Pacific (east Australian) Pomatomus saltatrix.

Pomatomus saltatrix is an important recreational fishing species with seven major populations worldwide. The reproductive biology of the southwest Pacific Ocean (east Australian) population is uncertain, with both an extended spawning and multiple spawning periods previously hypothesised. Here we demonstrate an altered sex ratio biased towards females and a larger length at 50% maturity (L50) compared to those recorded for the population 40 years ago, before comprehensive management strategies were implemented. We also report a second, previously undescribed, late-summer spawning event which was identified by analysing patterns in a gonadosomatic index across the whole population and an historical larval fish database. P. saltatrix are capable of spawning multiple times per season with estimates of batch fecundity ranging from 99,488 to 1,424,425 eggs per fish. When combined with the length frequency distribution of the population, the majority of eggs (64%) were shown to be produced by fish ≤40 cm fork length (FL). L50 was estimated at 30.2 and 31.5 cm FL for male and female P. saltatrix respectively, 4 cm larger than 40 years ago. The sex ratio of the population was found to have significantly shifted over the last 40 years from an equal sex ratio to a female dominated population (1.58 females:1 male). These dramatic alterations to the sex ratio and L50 highlights the value of monitoring the reproductive biology of exploited fish populations to ensure that management plans remain appropriate.

The influence of ontogenetic diet variation on consumption rate estimates: a marine example.

Consumption rates are the foundation of trophic ecology, yet bioenergetics models used to estimate these rates can lack realism by not incorporating the ontogeny of diet. We constructed a bioenergetics model of a marine predatory fish (tailor, Pomatomus saltatrix) that incorporated high-resolution ontogenetic diet variation, and compared consumption estimates to those derived from typical bioenergetics models that do not consider ontogenetic diet variation. We found tailor consumption was over- or under-estimated by ~5-25% when only including the most common prey item. This error was due to a positive relationship between mean prey energy density and predator body size. Since high-resolution diet data isn't always available, we also simulated how increasing dietary information progressively influenced consumption rate estimates. The greatest improvement in consumption rate estimates occurred when diet variation of 2-3 stanzas (1-2 juvenile stanzas, and adults) was included, with at least 5-6 most common prey types per stanza. We recommend increased emphasis on incorporating the ontogeny of diet and prey energy density in consumption rate estimates, especially for species with spatially segregated life stages or variable diets. A small-moderate increase in the resolution of dietary information can greatly benefit the accuracy of estimated consumption rates. We present a method of incorporating variable prey energy density into bioenergetics models.

Movements, Home Range and Site Fidelity of Snapper (Chrysophrys auratus) within a Temperate Marine Protected Area.

Understanding the movement dynamics of marine fish provides valuable information that can assist with species management, particularly regarding protection within marine protected areas (MPAs). We performed an acoustic tagging study implemented within the Port Stephens-Great Lakes Marine Park on the mid-north coast of New South Wales, Australia, to assess the movement patterns, home range and diel activity of snapper (Chrysophrys auratus; Sparidae); a species of significant recreational and commercial fishing importance in Australia. The study focused on C. auratus movements around Cabbage Tree Island, which is predominantly a no-take sanctuary zone (no fishing), with an array of acoustic stations deployed around the island and adjacent reefs and islands. Thirty C. auratus were tagged with internal acoustic tags in November 2010 with their movements recorded until September 2014. Both adult and juvenile C. auratus were observed to display strong site fidelity to Cabbage Tree Island with a mean 12-month residency index of 0.83 (range = 0 low to 1 high). Only three fish were detected on acoustic receivers away from Cabbage Tree Island, with one fish moving a considerable distance of ~ 290 kms over a short time frame (46 days). The longest period of residency recorded at the island was for three fish occurring regularly at the site for a period of 1249 days. Chrysophrys auratus displayed strong diurnal behaviour and detection frequency was significantly higher during the day than at night; however, there was no significant difference in detection frequency between different hours. This study demonstrates that even small-scale protected areas can benefit C. auratus during multiple life-history stages as it maintains a small home range and displays strong site fidelity over a period of 3 years.

Swim bladder function and buoyancy control in pink snapper (Pagrus auratus) and mulloway (Argyrosomus japonicus).

Physoclist fish are able to regulate their buoyancy by secreting gas into their hydrostatic organ, the swim bladder, as they descend through the water column and by resorbing gas from their swim bladder as they ascend. Physoclists are restricted in their vertical movements due to increases in swim bladder gas volume that occur as a result of a reduction in hydrostatic pressure, causing fish to become positively buoyant and risking swim bladder rupture. Buoyancy control, rates of swim bladder gas exchange and restrictions to vertical movements are little understood in marine teleosts. We used custom-built hyperbaric chambers and laboratory experiments to examine these aspects of physiology for two important fishing target species in southern Australia, pink snapper (Pagrus auratus) and mulloway (Argyrosomus japonicus). The swim bladders of pink snapper and mulloway averaged 4.2 and 4.9 % of their total body volumes, respectively. The density of pink snapper was not significantly different to the density of seawater (1.026 g/ml), whereas mulloway were significantly denser than seawater. Pink snapper secreted gas into their swim bladders at a rate of 0.027 ± 0.005 ml/kg/min (mean ± SE), almost 4 times faster than mulloway (0.007 ± 0.001 ml/kg/min). Rates of swim bladder gas resorption were 11 and 6 times faster than the rates of gas secretion for pink snapper and mulloway, respectively. Pink snapper resorbed swim bladder gas at a rate of 0.309 ± 0.069 ml/kg/min, 7 times faster than mulloway (0.044 ± 0.009 ml/kg/min). Rates of gas exchange were not affected by water pressure or water temperature over the ranges examined in either species. Pink snapper were able to acclimate to changes in hydrostatic pressure reasonably quickly when compared to other marine teleosts, taking approximately 27 h to refill their swim bladders from empty. Mulloway were able to acclimate at a much slower rate, taking approximately 99 h to refill their swim bladders. We estimated that the swim bladders of pink snapper and mulloway ruptured after decreases in ~2.5 and 2.75 times the hydrostatic pressure to which the fish were acclimated, respectively. Differences in buoyancy, gas exchange rates, limitations to vertical movements and acclimation times between the two species are discussed in terms of their differing behaviour and ecology.