In 2014, an unprecedented marine heatwave struck the West Coast of the United States, drastically altering the oceanic environment. This significant warming event led to widespread disruptions in marine life, with seabirds and sea lions notably affected. Observers reported numerous instances of emaciated animals washing ashore, highlighting a severe disturbance in the marine food web.
While initial concerns focused on the quantity of available food, scientists soon began to question the nutritional quality of the prey. This shift in focus prompted a team from UC San Diego, in collaboration with the Southwest Fisheries Science Center and UC Santa Cruz, to investigate the root causes of starvation in predators like the California sea lion, which suffered greatly during the heatwave.
Nutritional Quality of Ocean Prey
The research unveiled surprising findings regarding the nutritional value of common prey species such as anchovies, sardines, and squid. Although these species are staple foods for sea lions, their energy content varied significantly. In some cases, predators would need to consume vast quantities of lower-quality fish to meet their energy needs, a feat that is not feasible.
Stephanie Nehasil, a postdoctoral researcher at Stony Brook University, explained, “You could have two fish side-by-side that are the same size but have a huge range of variation in the amount of energy they provide.” This variability highlights the complexity of marine ecosystems, where size does not necessarily correlate with nutritional value.
Measuring Energy Content in Fish
To quantify the energy content of different fish, Nehasil and her team employed a bomb calorimeter, an instrument that measures the heat released from a small tissue sample. This method allowed them to assess the energy density of fish across various seasons, sizes, and regions.
“We saw a lot of mortality, which led us to question whether prey abundance or availability had changed during the event,” said Nehasil.
The study’s results revealed that even within a single fish species, energy values could fluctuate widely based on the fish’s age, maturity, and habitat.
Implications for Sea Lions and the Ecosystem
For California sea lions, the findings are particularly significant. During the heatwave, female sea lions struggled to find prey with sufficient energy to sustain themselves and their offspring, leading to high pup mortality rates. This situation raised critical questions about whether the issue was solely about food quantity or if food quality also played a crucial role.
Nehasil and her colleagues determined that while low-quality prey was plentiful, it was insufficient to support the sea lions’ dietary needs. This discovery challenges previous models that assumed uniform nutritional value among same-sized fish.
Why Old Models Fall Short
The study’s insights enhance the accuracy of bioenergetics models, which predict animal food requirements and ecosystem dynamics. These models are essential tools for fisheries management, marine mammal protection, and policy-making.
“This is another piece of information to help inform our models to get a better understanding of how these ecosystems work,” said Professor Carolyn Kurle, a co-author of the study.
By incorporating precise data on prey energy values, stakeholders can make more informed decisions, especially as climate change continues to exert pressure on marine ecosystems.
Ocean Changes and Fish Energy
The 2014 heatwave not only increased ocean temperatures but also disrupted the natural upwelling of cold, nutrient-rich waters. This upwelling is vital for sustaining the marine food chain, from plankton to large predators. Without it, the entire ecosystem weakens, affecting the energy density of fish.
Fish from nutrient-rich areas tend to grow larger and store more fat, providing more energy to predators. Conversely, fish from less productive regions are leaner and offer less nutritional value. This disparity poses a significant challenge for predators like sea lions, which face a shortage of high-energy prey.
“You have to use the currency of energy value to inform bioenergetics models,” emphasized Nehasil.
Energy Value of Ocean Prey
The research also involved local students from San Diego, who gained hands-on experience through a partnership with the Ocean Discovery Institute. This collaboration highlighted the importance of understanding prey energy values in assessing marine health.
Professor Kurle concluded, “We all want to have healthy ecosystems and there are many stakeholders who want these systems to succeed. We want everyone to have the best data possible to make important analyses and estimations.”
The full study was published in the Journal of Animal Ecology.
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