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Biology

Cooked fish: Ocean warming and global fisheries

Article: Cheung, W. W., Watson, R., & Pauly, D. (2013). Signature of ocean warming in global fisheries catch. Nature, 497(7449), 365-368. DOI:10.1038/nature12156

With warming water temperatures, fish and invertebrate species are expected to shift geographical distributions towards the poles and into deeper water.  More importantly, we expect global fisheries to change.

In a recent study by Cheung et al., an annual mean temperature of catch (MTC) metric was used to assess 52 marine ecosystems around the world from 1970 to 2006.  MTC is based on the inferred mean temperature, or the literature value for a species’ mean thermal preference, weighted by the annual catch.   For each marine ecosystem, a single MTC value was calculated based on the combined MTC of commercially fished species within the area.  The authors predicted that as stocks move towards higher latitudes, warm-water species catches would increase while cold-water species declined, leading to warming of the combined MTC.

This study found that commercial fisheries are indeed affected by changing species distributions, as evidenced by warming MTCs.  Furthermore, the rate of MTC warming differs by latitudinal range.  In the tropics, the extent of MTC warming is limited because after an initial decrease in the abundance of subtropical species –and increase in tropical species- the MTC would level off at the mean preferred temperature of tropical species.  Continued warming would eventually exceed tropical species’ physiological limits, resulting in decreased abundances; MTC would remain the same because the species assemblage would remain relatively unchanged (Figure 1).  A generalized additive mixed model was used to relate MTC to ocean warming and incorporate other factors such as oceanographic variability and fishing effort.

Figure1

Figure 1. Changes in catch species composition in relation to ocean warming and the resulting changes in MTC. Species distributions are related to ocean temperature (coloured bars) and temperature preferences of the exploited species (grey curves). Increase and decrease in abundance due to ocean warming are indicated by green curves and the reduction in area under the grey curves, respectively. The vertical black and red arrows represent MTC in the initial and subsequent decades, respectively. DMTC represents the difference in MTC relative to the initial decade. Species local extinction and invasion because of warming are indicated by red and green dotted curves, respectively. The expected changes in MTC over time are shown on the right.

Results

The models showed that overall, MTCs increased at 0.19°C per decade.  When tropical marine ecosystems were excluded, this jumped to 0.23°C per decade.  Furthermore, MTC increased in the northeast Pacific Ocean (0.48°C per decade) and northeast Atlantic Ocean (0.49°C per decade), with sea surface temperature (SST) increases of 0.20 and 0.26°C per decade, respectively.  As predicted, tropical ecosystems increased rapidly during the first decade but leveled off to about 0.26°C thereafter.  Subtropical species’ thermal tolerance ranges are wider than tropical species but with SST increasing steadily at about 0.14°C per decade, tropical ecosystems are becoming too warm to sustain subtropical species (Figure 3).

Figure3

Figure 3. Relationship between rates of change in MTC and SST between 1970 and 2006 in 52 marine ecosystems. a) Rate of change of MTC was calculated from the model in each ecosystem (slope of linear regression, P , 0.005, R2 5 0.19 (coefficient of determination)). The black line shows the mean and the grey lines delineate the 95% confidence interval. b) Changes in MTC (red) and SST anomalies (grey) in tropical ecosystems. The dashed lines are fitted with asymptotic and linear models for the MTC and SST anomalies, respectively.

A comparison of MTC calculated from catch data and relative abundances calculated from survey data supports that MTC is a valid metric for this assessment (Figure 4).  Fishing effort and SST simultaneously increased since the 1970s resulting in a correlation between the two factors in many marine ecosystems.  But, no evidence supports that fishing effort affects MTC.

The fact that this study was able to detect a catch response to climate change despite the potential for species’ adaptation to warming both at a phenotypic and evolutionary level suggests that the base relationship between ocean warming and MTC is strong.  Overall, marine fisheries catch is related to warming ocean temperatures.  As warming progresses, catches in higher latitudes will contain higher proportions of warm-water species.  Findings presented herein are particularly important to coastal fishing communities, more so in the tropics, where finite thermal tolerances will likely reduce future fish populations.

Figure 4. Comparison of MTC calculated from fisheries catch data and relative abundance calculated from scientific survey data. MTC is expressed as an anomaly relative to the mean of the time series, and relative abundance is expressed relative to the average between 1970 and 2006. The type of data used had no effect on the rate of change in MTC (ANCOVA, P . 0.1).

Figure 4. Comparison of MTC calculated from fisheries catch data and relative abundance calculated from scientific survey data. MTC is expressed as an anomaly relative to the mean of the time series, and relative abundance is expressed relative to the average between 1970 and 2006. The type of data used had no effect on the rate of change in MTC (ANCOVA, P . 0.1).

Further reading

Does this have you thinking about what the future holds for global fisheries?  In particular, what will happen in future warming scenarios?   Mills et al. (2013) examines the year 2012 as a basis for improving research and fisheries management policy.

Discussion

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  1. […] potential yield in high latitudes and decreases in the tropics were generally consistent with other studies using bioclimate envelope approaches.  However, when models were run for smaller spatial scales […]

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