Continuing with my Climate Change in Fisheries Series (CCFS), here is my 5th on Incorporating climate scenarios in fisheries research.
I was quite impressed with John Morrongiello's presentation from the University of Melbourne. It was nicely delivered, with some humour yet ultimate scientific rigour.
I'm used to reading a lot about the impact of climate change in fisheries at a macro scale. Yet, these changes start at individual and species levels and then cascade into stock changes, so it was refreshing to hear what is happening at the more granular level.
John’s presentation incorporated climate scenarios in fisheries research, covering methods for integrating climate scenarios into fisheries research design and included case studies demonstrating the application of scenarios in fisheries research.
The presentation explored the three key ways that ocean warming can impact fisheries.
Distributional changes,
reduced body size, and
alterations to the timing of key life history events (phenology).
It highlighted the extreme events, such as marine heatwaves, which are forecast to become more frequent and intense in a warmer future. These could result in key biological thresholds being surpassed sooner than would be expected based on the longer-term average sea surface temperature trend. Climate change will also likely cause shifts in the frequency and intensity of ENSO events.
It is important to consider a species’ ecological niche when exploring climate-driven distributional shifts. The ecological niche is the set of conditions in which a species survives and reproduces. These conditions determine where a species lives and how it responds to environmental changes, such as warming oceans.
Warming can allow species to expand their existing ranges as previously unfavourable areas become habitable. Range expansions can be driven by changes in the physical environment, or removal of other species that may have competitively excluded a species.
Warming can cause species to undergo a range shift, whereby their distribution tracks preferable conditions. Here, it might get too hot on the equatorial edge of a distribution and more favourable on the poleward edge of a distribution.
Warming can cause a range contraction. This usually occurs when conditions in part of a species’ range become unfavourable, but they lack the capacity to track these conditions further poleward.
Lastly, warming can cause changes in the depth distribution of species as they move deeper into cooler water. There are limits to the capacity of species to move deeper, driven by other aspects of their ecological niche. Two case studies, from waters around the USA and tuna in New Caledonia were discussed, to explore these different types of distributional changes.
Fisheries managers can consider the following points to help address the potential for distributional changes in their stocks:
Properly define stocks in the first place (using tagging, genetics etc.);
Monitor spatial distribution of stocks (including depth);
Be prepared to re-evaluate stock identification;
Be prepared to re-evaluate stock area; and
Be prepared to update stock models.
The ‘temperature-size rule’ describes the observed phenomena of increased juvenile growth, earlier maturation, reduced lifespan, and smaller adult size at higher temperatures. While the mechanisms underpinning the “temperature-size rule”remain debated, the implications of reduced body size on the viability of populations and productivity of fisheries are clear.
Of particular note is the fact that bigger fish produce disproportionately more offspring than smaller fish. Reductions in body size thus affect a stock's reproductive potential. Furthermore, smaller individuals often experience greater natural mortality due to predation.
Documenting changes in the body size of a stock can be difficult without a good time series of fish length/ weight data. One way to address this data shortage is to use the growth information naturally achieved in fish otoliths. Here, a few otolith collection events can be used to recreate the growth dynamics of a population over decadal scales. Climate signals can then be explored in these growth series.
Fisheries managers should, whenever possible:
Monitor the size and age structure of their stock’s catch (through time and/or space).
Consider targeted studies (e.g. otolith growth) to assess sensitivity of stock to current and future warming;
Don’t assume that life history parameters (e.g. age- or size-at-maturity) used in stock assessment models are stationary. Try to monitor these periodically (gonads or new maturity proxies);
Be prepared to update stock models with new knowledge; and
Promote the preservation of big fish.
Rapid warming can alter cues used by species to stimulate reproduction and developmental rates. If these shifts in reproduction or other life history events become out of sync with environmental conditions, significant impacts can occur on populations.
The match-mismatch hypothesis stipulates that recruitment is highest when newly hatched larvae encounter a food-rich environment. Advanced spawning timing that is not matched by advanced food availability causes recruitment failure.
In summary, climate change can impact fish and fisheries in a number of ways. In turn, fisheries managers must understand how warming will likely affect their stock. This could be through dedicated investigations or based on other existing knowledge.
Regardless, managers should continue to monitor their stocks as best as possible to ensure any changes in distribution, size, or phenology can be identified and appropriate actions taken to potentially offset productivity risks.
Discussion
The SST does not change much between day and night, however, if there is wind causing upwelling that will impact the water temperature over a small timeframe. As the ocean warms the metabolic rate of ectotherms increases, so larvae will grow faster and increase the rate at which they use of their yolk stores, if there is not enough food at that time as a result of lower zooplankton levels, they will starve. If the temperature goes too high that can also cause mortality, or for survivors who were living for prolonged periods in suboptimal conditions, there can be other impacts, such as a reduction in the immune system.
A good complex model allows you to explore many linked things, but you also run the risk of getting too complex, which can take a long time to perfect, and of ignoring the simpler models in the pursuit of perfection, and you may never get answers.
Fish weight is important to measure, but weight fluctuates more than length and is harder to measure, so models tend to use length data.
Rivers are influenced by rainfall, and big storm events impact rivers. These will impact the areas close to river mouths, estuaries, or mangroves but tend not to be that influential over a larger scale. They can, however, have some locally important influences.