Understanding Baltic Herring Age and Growth
Estimating the age of Baltic herring is typically done using otoliths, the earbones of fish. Until 1997, whole otoliths were used for herring from the Archipelago Sea. However, slow growth made it challenging to distinguish annual rings. From 1998 onwards, a new method was adopted. Each otolith is now fixed onto a polycarbonate chip, embedded in epoxy glue, and then ground to reveal the nucleus. This process improves visibility and allows for accurate counting of annual rings.
Life Stages and Longevity
Baltic herring typically reaches sexual maturity at age 3, with some individuals maturing at age 2. These fish have a long lifespan, living anywhere from 10 to 15 years, and in recent times, even beyond 20 years in the Airisto spawning area.
Diverse Spawning Population
The spawning population consists of herring of various ages. However, the dominant spawners are typically between 3 to 6 years old. Consequently, spawning shoals consist of fish of different sizes, with the smallest reproducing herring measuring around 10-11 cm.
Changes Over Time
Since 1984, we’ve been monitoring the spawning herring population in the Airisto area. Our data reveals that the herring population’s characteristics aren’t static and can shift with changing environmental conditions. Factors like decreasing salinity and fluctuating water temperatures have played a role.
Evolution of Size and Lipid Content
Our time-series data indicates a gradual decrease in the average size of the herring population, from 21 cm to 16 cm, while the average age remains stable. This size change is primarily due to a decline in herring growth rate, observed in various parts of the Baltic Sea. Additionally, lipid content in both ovaries and the whole fish has significantly decreased.
Influences on Lipid Content
Lipid content changes appear linked to salinity and winter temperature. Female length had no association with the ovarian lipid concentration that follows the general pattern found in many other fish species. The role of food resources in herring lipid reserves is evident, but we require more data on prey species abundance, especially of mysid shrimps and amphipods, critical to herring diets during autumn when lipid accumulation occurs.
Our findings indicate a gradual shift toward smaller herring, possibly favoring slow-growing individuals that enhance egg quality. This shift in body size may be an adaptive response to changing environmental conditions.
Herring spawning occurs primarily from May to July, though it can start as early as April and last until August. Seasonal variation, as observed in 1990, impacts herring characteristics like gonad weight and fecundity. This seasonal variation is essential to consider when comparing samples from different times or areas.
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