📄 earth_science

Quantifying prey energetic quality: energy density and size-energy relationships for marine fish and cephalopods

This study provides contemporary energy density and size-energy relationships for 34 North Sea fish and two cephalopod species using bomb calorimetry, offering critical data to update ecological models of predator-prey dynamics and assess the impacts of environmental change on marine food webs.

Original authors: Philippa Wright, Cormac Booth, Katherine Whyte, Laura Oller, Charlie Cooper, Thomas Régnier, Sophie Smout, Gordon Hastie

Published 2026-08-10

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CC BY 4.0

Original authors: Philippa Wright, Cormac Booth, Katherine Whyte, Laura Oller, Charlie Cooper, Thomas Régnier, Sophie Smout, Gordon Hastie

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the ocean as a giant, bustling city where every creature is constantly on the hunt for its next meal. In this underwater metropolis, energy is the currency. Just like you need calories to run, play, and grow, marine animals need energy to survive the cold water, swim against currents, and raise their families. But not all meals are created equal. A sandwich made of dense, fatty peanut butter packs a much bigger punch than a sandwich made of fluffy, watery lettuce, even if they look the same size. Scientists call this "energy density"—how much fuel is packed into a single gram of food. For the top predators of the sea, like seals, whales, and seabirds, knowing which fish are the "peanut butter" and which are the "lettuce" is a matter of life and death. If they pick the wrong menu, they might starve even with a full belly.

This is where a team of researchers from the University of St Andrews and the Marine Directorate stepped in. They wanted to update the "menu" for the North Sea, a busy underwater neighborhood off the coast of Scotland. For a long time, scientists had to guess how much energy different fish provided, often relying on old data that might not fit the changing ocean today. The team decided to stop guessing and start measuring. They used a high-tech tool called a bomb calorimeter—think of it as a super-precise furnace that burns a tiny piece of fish to measure exactly how much heat (energy) it releases. By doing this for dozens of species, from tiny sandeels to large cod, they built a fresh, detailed map of who is eating what, and exactly how much fuel each bite provides.

The Big Fish, The Small Fish, and The Energy Puzzle

The researchers collected hundreds of fish and squid specimens from the North Sea, mostly between June and October. They measured every single one, from their snout to their tail, and then put them through the "furnace" test. The goal was to answer two big questions: How much energy does a fish have per gram (energy density), and how does the total energy in a fish change as it grows bigger?

The results were a bit of a surprise. The team found that energy density varies wildly depending on the species. The "peanut butter" of the sea turned out to be the Clupeidae family, which includes herring, sprat, and mackerel. These fish are loaded with fat and pack a massive energy punch, with herring reaching up to 7.13 kJ g-1 on average. On the other end of the spectrum, the "lettuce" fish were found in families like Gadidae (which includes haddock and whiting) and Callionymidae (dragonets). These fish are much leaner, with energy densities hovering around 3.74 kJ g-1 to 4.40 kJ g-1.

One of the most interesting discoveries was about timing. The researchers looked closely at Raitt's sandeel, a tiny fish that is a favorite snack for many seabirds and seals. They found that these fish are not the same year-round. A sandeel caught in the winter (December) was significantly "heavier" in energy than one caught in the summer (June-July). A 15 cm sandeel in winter contained about 136.8 kJ of energy, while a sandeel of the exact same size in summer only had 66.8 kJ. It's like the winter fish had been stuffing itself with snacks before hibernating, while the summer fish was still on a diet. This seasonal shift means that predators need to work much harder in the summer to get the same amount of energy they get easily in the winter.

Size Matters (A Lot)

The study also looked at how the total energy in a fish changes as it grows. The answer is simple but powerful: bigger fish are exponentially better meals. The relationship isn't a straight line; it's a curve that shoots up fast. For example, a 15 cm haddock has about four times the total energy of a 10 cm haddock. For a predator, catching one big fish is often much more efficient than catching three small ones.

The researchers tried to see if there was a specific pattern where energy density goes up as fish get bigger (like a sigmoidal curve), which some older studies suggested. However, for most of the species they tested, that pattern didn't show up. The data was too messy and varied to fit that specific mathematical shape. This suggests that for many of these fish, being bigger doesn't automatically mean being "fatter" or more energy-dense per gram; it just means there is more of the fish to eat.

Why This Matters for the Ocean

This new data is like a fresh, accurate nutrition label for the North Sea. It helps scientists understand how predators like seals and puffins are faring in a changing world. If the ocean gets warmer and the "peanut butter" fish move away, leaving only the "lettuce" fish behind, predators might have to eat twice as many fish just to survive. The study also highlights that a tiny difference in size can make a huge difference in energy. A sandeel that is just 0.5 cm shorter than its neighbor can contain one-third less energy. This means that if the average size of the fish population drops, the predators could face an energy crisis even if there are plenty of fish around.

By providing these updated numbers, the study gives us a better tool to predict how marine life will cope with human impacts, like fishing or offshore wind farms, and climate change. It reminds us that in the ocean, it's not just about how many fish are in the water, but exactly how much fuel each one carries.

Technical Summary: Quantifying Prey Energetic Quality in North Sea Marine Fish and Cephalopods

Problem Statement
Marine top predators, including pinnipeds and seabirds, rely on the energy content of prey to determine foraging profitability, survival, and reproductive success. While energy density (kJ g⁻¹) is a critical metric for understanding trophic transfer and ecosystem resilience, there is a significant paucity of contemporary data regarding the energy content of prey species in the North Sea. Existing studies often rely on generalized or historical values that fail to account for current environmental conditions, regional variations, or seasonal fluctuations. Furthermore, the relationship between prey size and total energy content is not well-characterized for many non-commercial species, creating uncertainty in predator-prey dynamic models and assessments of anthropogenic impacts.

Methodology
This study addressed these data gaps by conducting prey sampling surveys in coastal regions of the North Sea (Moray Firth, Firth of Forth, and Firth of Tay) between 2019 and 2024. Specimens were opportunistically collected from demersal and pelagic trawls, modified scallop dredges, and baited fish traps used to validate independent fisheries acoustic surveys. No animals were captured specifically for this research; all individuals were dead upon retrieval.

The study focused on 34 fish species and two cephalopod species. For each specimen, total length and mass were measured. Samples were homogenized, dried to a constant mass at 60°C to determine moisture content, and processed into pellets. Energy density was measured using bomb calorimetry (Parr 6200 Isoperibol Calorimeter), with wet weight energy density calculated by adjusting dry mass energy density for moisture content.

Statistical analyses were performed in R to model:

Length-Weight Relationships: Fitted using an allometric model ( $W = a_{LW}L^{b_{LW}}$ ) via Nonlinear Least Squares (NLS).
Length-Energy Density Relationships: Tested against a logistic (sigmoidal) model previously proposed in literature to detect nonlinear increases in energy density with size.
Length-Total Energy Relationships: Fitted using an allometric model ( $E_T = a_{LE}L^{b_{LE}}$ ) where total energy is a function of length.

Species with fewer than four data points or restricted length ranges were excluded from modeling but included in mean energy density calculations. Seasonal comparisons were made specifically for Raitt's sandeel (Ammodytes marinus), which was sampled in both summer and winter.

Key Results

Dataset Generation: The study produced 601 new energy estimates for 36 species. Mean energy densities ranged from 3.59 kJ g⁻¹ (Callionymidae) to 7.30 kJ g⁻¹ (Clupeidae).
Species Variation: High energy density was observed in lipid-rich pelagic species such as mackerel, herring, and sprat. Lower energy densities were found in families such as Gadidae, Cottidae, and Pleuronectidae.
Length-Energy Density Relationship: Contrary to previous studies suggesting a sigmoidal increase in energy density with length (often linked to reproductive maturity), the logistic NLS models failed to converge for all species in this dataset. The authors attribute this to the sampling window (summer/autumn, post-spawning) and restricted size ranges, which likely lacked the specific physiological states (e.g., pre-spawning lipid accumulation) required to drive the nonlinear trend. Herring was an exception showing a positive trend, though the model did not converge, likely due to sample size constraints.
Length-Total Energy Relationship: A clear, positive exponential relationship was established between fish length and total energy content for 21 species. The rate of energy increase varied significantly by species, driven by species-specific length-weight and energy density relationships.
Seasonal Dynamics: Raitt's sandeel exhibited significant seasonal variation. Winter specimens showed higher energy density (6.54 kJ g⁻¹) and a steeper length-weight growth coefficient compared to summer specimens (6.01 kJ g⁻¹). Consequently, a 15 cm sandeel in winter contained approximately double the total energy of a summer specimen of the same size.
Trophic Levels: Forage fish demonstrated significantly higher mean energy density (5.44 kJ g⁻¹) compared to predatory fish (4.90 kJ g⁻¹).

Significance and Claims
The authors position this work as a critical update to historical data, providing a contemporary resource for ecological studies in the North Sea. The primary significance lies in:

Filling Data Gaps: Providing the first published energy estimates for several non-commercial species (e.g., common goby, snake blenny) that serve as potential prey for marine mammals and seabirds.
Refining Predator-Prey Models: The generated length-total energy relationships allow for more accurate parameterization of dynamic energy budget models used to predict predator responses to changes in prey availability and size distributions.
Highlighting Temporal Context: The study demonstrates that seasonal context is vital; for species like Raitt's sandeel, winter energy reserves are significantly higher, impacting the energetic balance of predators during different times of the year.
Application to Anthropogenic Impacts: The dataset is intended to support assessments of how climate change and industrial developments (such as offshore wind farms) may alter fish assemblages and, subsequently, the energy flow through coastal marine food webs.

The paper concludes that while energy density varies widely, the total energy content of prey is the more deterministic factor for predator intake, and small shifts in prey size can result in substantial changes in the number of prey items required to meet energetic demands. These findings contribute to a better understanding of energy flow in coastal marine ecosystems and the resilience of top predators to environmental change.

The Big Fish, The Small Fish, and The Energy Puzzle

Size Matters (A Lot)

Why This Matters for the Ocean

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