Jock Young - Academia.edu (original) (raw)

Papers by Jock Young

A total of 1423 stomach samples were taken from Patagonian toothfish, Dissostichus eleginoides, c... more A total of 1423 stomach samples were taken from Patagonian toothfish, Dissostichus eleginoides, caught by bottom trawls at two fishing grounds near Macquarie I., over three fishing seasons. Fish were caught at depths ranging from 500 to 1290 m, and ranged in size from 310 to 1490 mm total length. The 462 stomach samples (32%) that contained prey items indicated that toothfish preyed on a broad range of species including fish, cephalopods and crustaceans (58%, 32% and 10% biomass, respectively), suggesting that they are opportunistic predators. The bathypelagic fish Bathylagus sp. was the most important fish prey (14% dietary biomass); however, nototheniid, macrourid, morid and myctophid fish were also taken. The squid Gonatus antarcticus was also an important prey species (16% biomass), and many other cephalopod species were taken in low frequency. Prawnlike crustaceans (Nematocarcinidae, Mysididae, Sergestidae and Euphausiidae) were the most important crustaceans taken (9% of prey biomass). Significant inter-seasonal and inter-fishing-ground differences in diet were found, but dietary composition was not related to fishing depth, fish size (with the exception of one fishing ground in one season) or the time of day of capture. Comparison with other studies reveals biogeographical differences in the diet of toothfish. D i e t o f P a t a g o n i a n t o o t h f i s h S . D . Go l d s w o r t h y e t a l . M F 0 0 0 7 5

Reviews in Fish Biology and Fisheries, Jan 1, 2010

Pelagic longline fisheries target (or catch incidently) large apex predators in the open ocean (e... more Pelagic longline fisheries target (or catch incidently) large apex predators in the open ocean (e.g. tunas, billfish and sharks) and have the potential to disrupt the ecosystem functionality if these predators exert strong top–down control. In contrast, warming of oceans from climate change may increase bottom–up effects from increases in primary productivity. An ecosystem model of a large pelagic ecosystem off eastern Australia was constructed to explore the potential ecological effects of climate change and longlining by Australia’s Eastern Tuna and Billfish Fishery. The model reproduced historic biomass and fishery catch trends from 1952 to 2006 for seven functional groups. Simulated changes in fishing effort and fishing mortality rate on individual target species from 2008 to 2018 resulted in only modest (<20%) changes in the biomass of target species and their direct predators or competitors. A simulated increase in phytoplankton biomass due to climate change resulted in only small increases (<11%) in the biomass of all groups. However, climate-related changes to the biomass of micronekton fish (−20%) and cephalopods (+50%) resulted in trophic cascades. Our results suggest there may be ecological redundancy among high trophic level predators since they share a diverse suite of prey and collectively only represent <1% of the total system biomass. In contrast, micronekton fishes and cephalopods have high biomasses and high production and consumption rates and are important as both prey and predators. They appear to exert ‘wasp–waist’ control of the ecosystem rather than top–down or bottom–up processes reported to drive other pelagic systems.

Background Modern fisheries management encompasses ecosystem-wide perspectives, and sustainabilit... more Background Modern fisheries management encompasses ecosystem-wide perspectives, and sustainability is measured both in terms of the target stocks but also in terms of the direct and indirect impact that fishing practices may have upon populations of non-target, associated and dependent species and ecosystem structure and function (Garcia and Cochrane 2005).

Deep Sea Research Part II: Topical Studies in Oceanography, 2011

Climate change presents an emerging challenge to the sustainable management of tuna fisheries, an... more Climate change presents an emerging challenge to the sustainable management of tuna fisheries, and robust information is essential to ensure future sustainability. Climate and harvest affect tuna stocks, populations of non-target, dependent species and the ecosystem. To provide relevant advice we need an improved understanding of oceanic ecosystems and better data to parameterise the models that forecast the impacts of climate change. Currently oceanwide data collection in the Pacific Ocean is primarily restricted to oceanographic data. However, the fisheries observer programs that operate in the region offer an opportunity to collect the additional information on the mid and upper trophic levels of the ecosystem that is necessary to complement this physical data, including time-series of distribution, abundance,

Journal of Plankton Research

Information on the diets of marine fish larvae found in Australian waters is scarce, particularly... more Information on the diets of marine fish larvae found in Australian waters is scarce, particularly when we consider that of some 3000 species of fishes in the waters of the Australian Fishing Zone, there are published data on feeding for fewer than twenty species. In the larvae studied so far, diets are dominated by the various life history stages of copepods, although other taxa (including bivalve veligers, appendicularians, tintinnids and other fish larvae) dominate the diets of some species. Generally, feeding success, prey size and prey diversity all increase as the larvae grow. Nevertheless, flexibility and opportunism in feeding are important characteristics of larval behaviour. Larvae generally feed during the day, with feeding peaks in the morning and afternoon, although some species feed throughout the day and some feed only at night. Gut evacuation rates range from 2 to 6 h. Larvae eat between 10 and 30% of their body weight (or 5 to 100 prey) per day, depending on prey type and availability. Larvae in tropical oligotrophic waters can affect the abundance of their prey, which results in competition for food and subsequent density-dependent reduction in growth rate. This contrasts with larvae from temperate waters, which do not appear to be food-limited. Present studies indicate that marine fish larvae in Australian waters have similar feeding strategies to their counterparts elsewhere in the world. However, with so few larval species examined in detail, such generalisations are premature. Future research should aim to 1) collect larval feeding data on more species and 2) examine further the relationship between feeding ecology, recruitment success and seasonal and interannual variations in plankton production.

The reproductive ecology of broadbill swordfish, Xiphias gladius, was evaluated based on 1437 fis... more The reproductive ecology of broadbill swordfish, Xiphias gladius, was evaluated based on 1437 fish (size range 50-300 cm orbital fork length [OFL]) caught in the domestic longline fisheries off eastern Australia and New Zealand between May 1999 and March 2001. Reproductive activity was assessed using histology, a gonadosomatic index, and maximum oocyte size. Males were significantly smaller than females and represented less than one third of the sampled fish. Sex ratio differed significantly with respect to fish size and time of year. Females began maturing at 150-cm OFL and spawned from September to March, with the greatest activity from December to February. Males matured at 90-cm OFL; ripe males were found from January to March, but also in May and October, suggesting an extended reproductive period. During the spawning period the proportion of spawning to inactive mature sized females was significantly higher in waters west of longitude 158 • E than in waters to the east. Further to the east, samples taken from the New Zealand fishery showed no actively spawning fish during the main spawning period. Females were increasingly reproductively active as water temperature increased beyond 24 • C and sea surface chlorophyll a decreased below 0.2 µg L −1 . Batch fecundity was linearly related to fish length with a mean batch fecundity of 1.66 million oocytes for females ranging in size from 173-to 232-cm OFL. The presence of hydrated oocytes and post-ovulatory follicles (POFs) in the same ovaries indicated multiple spawnings. Depending on the time taken for POFs to degrade, these may have been daily at the height of the spawning season.

Deep-sea Research Part Ii-topical Studies in Oceanography, 2011

Although many species in the pelagic ocean are widespread, they are not randomly distributed. The... more Although many species in the pelagic ocean are widespread, they are not randomly distributed. These species may have associations with particular water masses or habitats, but to best understand patterns in the ocean, these habitats must be identified. Previous efforts have produced static or seasonal climatologies, which still represent smearing over habitats. The Eastern Tuna and Billfish Longline Fishery (ETBF) targets a range of high trophic level species in oceanic waters off eastern Australia. In this study, dynamic ocean habitats in the region were identified for each month based on cluster analysis of five oceanographic variables averaged at a monthly time scale and a spatial scale of 0.5° for the period 1995-2006. A total of seven persistent habitats were identified off eastern Australia with intra and interannual variation in size and location, indicating the importance of spatial and temporal variation in the dynamics of the region. The degree to which these dynamic habitats were distinguished was tested using (i) stable isotope analysis of top fish predators caught in the region and (ii) estimates of variation in estimated abundance generated from catch data from the fishery. More precise estimates (measured as lower total CV) of isotopic values from swordfish ( Xiphias gladius), yellowfin tuna ( Thunnus albacares) and albacore ( Thunnus alalunga) were obtained for 4 of 6 isotope comparisons using the dynamic habitat groupings, which indicate that stratifying by pelagic habitat improved precision. Dynamic habitats produced more precise abundance estimates for 7 of 8 large pelagic species examined, with an average reduction in total CV of 19% compared to when abundance was estimated based on static habitat stratification. These findings could be used to guide development of effective monitoring strategies that can distinguish patterns due to environmental variation, and in the longer term, climate change.

half century, leading some to hypothesize their abundance has increased due to depletion of their... more half century, leading some to hypothesize their abundance has increased due to depletion of their predators. Landings data are confounded, however, by changes in targeting practices which themselves are driven by the creation and dissipation of markets for fisheries products. There are few long-term surveys of cephalopod productions that cover the entire ranges of populations. We are using the pelagic-predators-diet data base to consider whether feeding rates on cephalopods, principally squids, have increased from the mid-twentieth century to present. Our initial assessment has revealed substantial increases in the frequency of occurrence of squid in all apex predator diets over the past half century and may provide the first direct empirical evidence for large-scale increase in cephalopod populations.