Nguyen, Hoang Minh, et al. “Growth of a deep-water, predatory fish is influenced by the productivity of a boundary current system.” Scientific reports. 5 (2015). doi:10.1038/srep09044
Background and Methods
Bottom-dwelling fishes living in the deep waters at the edges of continental shelves (around 200m, or 656 ft.) may be particularly vulnerable to projected conditions associated with climate change in the next 50 to 100 years. Living near the sea bottom may limit ability to move around physical barriers in response to changing environmental conditions. Most of these fish are heavily exploited in commercial fisheries, leaving populations in an over-harvested state with limited resilience to environmental pressure.
Most of the deep, bottom-dwelling fish are considered secondary or apex predators. Secondary or apex predators feed at a higher trophic level. This means that the energy that is produced from photosynthesis as primary production (the lowest trophic level) must pass through several organisms before it is consumed by deep-sea predatory fish. Linkages between trophic levels are not always linear. Therefore the flow of energy from lower trophic levels may be obscured by complex trophic interactions thus making it difficult to connect the fish to climate change effects on primary production.
Several studies have linked warming sea surface temperatures to changing fish distributions, and climate change is also predicted to modify ocean circulation patterns which is likely to affect primary production. It is difficult to predict the impacts of combined effects of climate change to systems, in part due to lack of long term data. Previous studies indicate fish growth rates may be derived from rings in otolith bones, similar to reading tree rings (Figure 1). This method allows the combination of growth records from many fish into a single time series that provides a population level growth rate response to environmental drivers.
This study assessed the growth rates of hapuku (Polyprion oxygeneios), a large fish (up to 175cm) which is found around 200 to 850m. The study sampled hapuku 27 to 49 years of age, captured along the Southwestern coast of Australia (Figure 2). There, the Leeuwin Current is projected to slow down under the progression of climate change. This current is a strong influence on the productivity of the region and may therefore have impacts on higher trophic level feeders like hapuku.
Results and Significance
Individual otolith growth rates were compiled into a mean index chronology for 1990-2004, a 15-year period (Figure 2). Mean annual growth rate was positively correlated with mean annual sea level at Fremantle from the prior year. This means that a higher (lower) annual sea level in one year was related to a faster (slower) growth rate in the following year. Mean annual sea level at Fremantle is considered a proxy for strength of the Leeuwin Current, which is typically associated with warm and cold core eddies in austral autumn and winter. These eddies mix the water column, bringing rich nutrients from the bottom up to the surface and increasing primary productivity beginning in March. Thus faster growth rates are likely related to the strength of the Leeuwin, because it increases the amount of food energy available to reach higher trophic levels like apex predators. The one-year delay between sea level and growth rate is probably due to the amount of time required for energy to be assimilated in the higher trophic levels.
Gut content analyses indicate that the hapuku feed primarily on squid, which have strong daily vertical migrations from the surface waters to depth, where hapuku live. This makes for a fairly simple food chain between surface waters and the hapuku at depth. Squid grow relatively quickly and therefore reflect changes in the environment rapidly. The lag between hapuku growth and the Leeuwin current may reflect the time required for squid to reach large enough size for hapuku to prey on them.
Hapuku growth was negatively correlated with the Multivariate El Niño/Southern Oscillation Index (MEI) of the previous year and positively correlated with sea surface temperatures (SST). This means that growth rates were greater following La Niña years and warmer SST. The results makes sense given that the Leeuwin current is strengthened in La Niña years and warmer temperatures are associated with a stronger current.
Models of climate change impacts on regional oceanography predict that the Leeuwin Current will decrease over the next 100 years as ocean temperatures warm. By the year 2060, the current strength may decline by as much as 15%. This has negative implications for deep sea predators like hapuku which appear to depend on the primary production generated by the current. Under the results of this study, we can expect fisheries for deep sea predators like hapuku to be negatively impacted by declining growth rates.