Periodic Reporting for period 2 - OCEAN DEOXYFISH (Ocean deoxygenation effects on threatened top predators: New understanding and predictions from novel bio-logging instruments and data)
Okres sprawozdawczy: 2022-04-01 do 2023-09-30
Models predict climate-driven ocean warming and reduced ventilation of the deep ocean will lower levels of dissolved oxygen (DO) in the ocean by 1–7% by 2100, with in situ observations suggesting reductions of >2% since 1960. Globally distributed, open-ocean oxygen minimum zones (OMZ) and ‘dead zones’ are increasing in area and volume as a result, with unknown consequences for ecosystems and biodiversity. Potential impacts may be particularly significant for large, fast-swimming oceanic predators with high oxygen demands, such as sharks and tunas, by reducing available habitats and concentrating them further in surface waters where they are more vulnerable to fisheries. A major concern is how ocean warming, deoxygenation and fisheries exploitation may interact to impact future sustainability of apex predator populations. However, increased vulnerability of oceanic predators from surface fisheries due to expanding OMZs has not been quantified because direct measurements in OMZ habitats have not been made; new approaches and data are needed.
This proposal addresses major unknowns in oceanic fish behaviour and ecology in oxygen poor environments that will predict responses to future warming and ocean deoxygenation and the effects on species vulnerability to fishing. We apply individual-based tracking studies and movement analysis and develop new technologies to understand from direct measurements how oceanic fish actually respond to hypoxic regions. Modelling studies establish the effects of future warming and deoxygenation on predator niches, shifting distributions and altered susceptibility to fishing exploitation.
Specific objectives are to (1) determine broad-scale space use and fine-scale physiological and behavioural responses of oceanic fishes to OMZs to quantify biophysical habitat preferences and test the habitat compression hypothesis directly; (2) quantify oceanic fish metabolism and predator-prey interactions directly to determine the ecological importance of waters around OMZs to oceanic fishes for testing the habitat trap hypothesis; (3) quantify spatial overlap between remotely tracked fish and fishing vessels to determine the impact of biophysical habitat changes on susceptibility to fisheries above OMZs and adjacent areas; and (4) develop and test spatially explicit models based on empirical fish-habitat-vessel relationships to predict future habitat availability under climate-driven OMZ expansion and fisheries exploitation.
This proposal addresses major unknowns in oceanic fish behaviour and ecology in oxygen poor environments that will predict responses to future warming and ocean deoxygenation and the effects on species vulnerability to fishing. We apply individual-based tracking studies and movement analysis and develop new technologies to understand from direct measurements how oceanic fish actually respond to hypoxic regions. Modelling studies establish the effects of future warming and deoxygenation on predator niches, shifting distributions and altered susceptibility to fishing exploitation.
Specific objectives are to (1) determine broad-scale space use and fine-scale physiological and behavioural responses of oceanic fishes to OMZs to quantify biophysical habitat preferences and test the habitat compression hypothesis directly; (2) quantify oceanic fish metabolism and predator-prey interactions directly to determine the ecological importance of waters around OMZs to oceanic fishes for testing the habitat trap hypothesis; (3) quantify spatial overlap between remotely tracked fish and fishing vessels to determine the impact of biophysical habitat changes on susceptibility to fisheries above OMZs and adjacent areas; and (4) develop and test spatially explicit models based on empirical fish-habitat-vessel relationships to predict future habitat availability under climate-driven OMZ expansion and fisheries exploitation.
Behaviour tracking and new DO tag development: We determined broad-scale space-use and behavioural responses of oceanic fishes to low DO (Objective 1) by satellite tracked movements and recorded diving depths of pelagic sharks in the eastern tropical Atlantic OMZ and in adjacent areas for comparison. Results so far show blue shark average maximum dive depth in the OMZ was 40% less than the mean depth attained outside, together with reduced deep diving frequency below 600 m inside the OMZ. This supports the habitat compression hypothesis (reduced vertical extent). To determine the physiological responses of sharks to DO we have fully developed, tested and are currently deploying on sharks a new animal-attached Dissolved Oxygen Measuring (DOME) archival tag.
Quantifying shark movements and metabolism: To determine oceanic fish metabolism and predator-prey interactions directly (Objective 2) we have deployed animal-attached accelerometers and a newly developed multi-sensor tag with a video camera onto pelagic sharks. Results so far have quantified cruising and burst swimming speeds and estimated field metabolic rates that will be used for calculating energy expenditure in different thermal habitats of the OMZ.
Overlap of fish and fishing vessels: We determined the spatial overlap between remotely tracked fish and satellite-tracked fishing vessels to quantify how biophysical habitat changes of OMZs affected fishing activity and shark mortality (Objective 3). Results show there was greater intensity of longline fishing effort above the eastern tropical Atlantic OMZ compared to adjacent waters. Higher shark retained catches (shark mortality) were associated with strong DO gradients, suggesting potential aggregation along suitable DO gradients contributed to habitat compression and higher fishing-induced mortality.
Modelling fish-habitat relationships: We developed spatially explicit models based on empirical fish-habitat relationships to determine drivers of shark habitat compression (Objective 4). Environmental modelling OMZ showed shark maximum dive depths decreased due to combined effects of decreasing DO at depth, high sea surface temperatures, and increased surface-layer net primary production. Multiple factors associated with climate-driven deoxygenation contributed to blue shark vertical habitat compression, which we showed increased shark mortality due to surface fisheries.
Quantifying shark movements and metabolism: To determine oceanic fish metabolism and predator-prey interactions directly (Objective 2) we have deployed animal-attached accelerometers and a newly developed multi-sensor tag with a video camera onto pelagic sharks. Results so far have quantified cruising and burst swimming speeds and estimated field metabolic rates that will be used for calculating energy expenditure in different thermal habitats of the OMZ.
Overlap of fish and fishing vessels: We determined the spatial overlap between remotely tracked fish and satellite-tracked fishing vessels to quantify how biophysical habitat changes of OMZs affected fishing activity and shark mortality (Objective 3). Results show there was greater intensity of longline fishing effort above the eastern tropical Atlantic OMZ compared to adjacent waters. Higher shark retained catches (shark mortality) were associated with strong DO gradients, suggesting potential aggregation along suitable DO gradients contributed to habitat compression and higher fishing-induced mortality.
Modelling fish-habitat relationships: We developed spatially explicit models based on empirical fish-habitat relationships to determine drivers of shark habitat compression (Objective 4). Environmental modelling OMZ showed shark maximum dive depths decreased due to combined effects of decreasing DO at depth, high sea surface temperatures, and increased surface-layer net primary production. Multiple factors associated with climate-driven deoxygenation contributed to blue shark vertical habitat compression, which we showed increased shark mortality due to surface fisheries.
The project will progress to provide new information on the broad-scale, long-term utilisation of OMZs by diverse oceanic fish. We have established new technologies and ongoing capability to measure direct responses of free-ranging fish to DO environments and to record prey fields and predator-prey interactions directly in relation to OMZ DO gradients. This enables the first, quantitative field tests of the habitat compression hypothesis and quantification of energy budgets of oceanic fish utilising waters above OMZs to determine ecological significance and to support the first field tests of the habitat trap hypothesis. Furthermore it will provide new O2–depth profiles for hydrographic databases and models by using fish as ‘animal oceanographers’.
The project will provide determination of spatial overlap (interactions) between oceanic fish and fishing vessels around OMZs and quantify relationships between fishing intensity and subsurface O2 concentrations. Provision of horizontal and vertical risk maps to identify fish vulnerability above OMZ environments will complement modelling predictions of fish habitat compression in relation to future expansion of the OMZ. These outputs will be used to identify global oceanic fish vulnerability hotspots arising from interactions between OMZ expansions and fishing intensity.
The project will provide determination of spatial overlap (interactions) between oceanic fish and fishing vessels around OMZs and quantify relationships between fishing intensity and subsurface O2 concentrations. Provision of horizontal and vertical risk maps to identify fish vulnerability above OMZ environments will complement modelling predictions of fish habitat compression in relation to future expansion of the OMZ. These outputs will be used to identify global oceanic fish vulnerability hotspots arising from interactions between OMZ expansions and fishing intensity.