Details are presented in a study led by the ICM-CSIC that links size differences in sardines between two very close areas with the water temperature in which the populations live.
A new study led by the Institut de Ciències del Mar of the CSIC (ICM-CSIC) has identified temperature as a key factor in the reduced growth of sardine (Sardina pilchardus) in the north of the Catalan coast. The research shows notable differences in sardine size between two very close areas, Arenys de Mar (south) and the Gulf of Roses (north), which are explained by different temperature conditions. The article, recently published in the journal Marine Environmental Research, provides new information to better understand the sardine crisis in the north of the Catalan coast and in the Gulf of Lions.
The research is part of the WINFISH project, focused on analyzing how environmental conditions affect temperate-water fish species such as sardine. To carry it out, the team combined classical otolith reading techniques—calcified structures that record the age and growth of fish—with oceanographic data obtained by satellite, such as sea surface temperature or chlorophyll concentration, an indicator of primary productivity.
Clear growth differences
Results show that sardines from the south grow faster and reach larger sizes at the same age as those from the north, probably because they live in warmer waters during spring and summer, the species growth season.
“The northern area, adjacent to the Gulf of Lions, is strongly influenced by cold northerly winds, which make the water about 3 ºC colder during this key period,” explains Joan Mir, first author of the study. “This difference affects the initial growth of juveniles, which is limited from the first year and conditions their whole life.”
The team rules out that the differences are due to biological factors such as age of sexual maturity, body condition, reproductive period, or fishing pressure, since they are similar in both areas.
A crisis related to climate change
In addition, the research team analyzed how temperature and primary productivity have varied over time and detected a thermal regime shift in the north in 2007. This change coincided with the collapse of the sardine fishery in the Gulf of Lions and could explain the sudden worsening of individual growth in this region since then.
“It is worth noting that the northern area is influenced by oceanographic processes taking place in the Gulf of Lions, and we have observed that both the water temperature and the growth and size composition of sardines in this area are very similar to those of the Gulf of Lions,” comments Ana Sabatés, principal investigator of the project.
“Although on a small scale higher temperature implies greater growth, under climate change scenarios the situation is more complex,” notes Mir. “The sudden drop in sardine size in the Gulf of Lions could be explained by a reduction in growth between ages 1 and 2 from 2008 onward, probably related to an earlier age of maturity, triggered by the temperature increase.”
The study also reveals a phenological change in primary productivity, key for the feeding of juvenile sardines. In the northern area, the period of maximum productivity now ends 32 days earlier than 20 years ago, which represents a 25% reduction in days with high productivity, while in the south it has remained more stable:
“This double effect—higher temperature and decreased food—would generate considerable stress for young sardines, especially in the north,” highlights Ana Sabatés, principal investigator of the project.
Perspectives for management and research
The study’s results not only help to better understand the decline of sardines but also point to the need for more adaptive fisheries management, especially in the current context of global change.
“Knowing how populations respond to environmental conditions allows us to anticipate responses and adjust management policies to ensure the sustainability of resources,” notes the scientific team.
For future research, the goal will be to deepen the study of the physiological and metabolic mechanisms behind these changes, such as early maturation in adverse environmental contexts. This line of research may be key to understanding whether marine species will be able to adapt to increasingly extreme climate scenarios.