CORRELATION BETWEEN FISH SIZE AND OTOLITH SIZE IN ATLANTIC COD (GADUS MORHUA) AND ATLANTIC HERRING (CLUPEA HARENGUS): A comparison of growth patterns over four decades - in northern Skagerrak

Abstract

Atlantic cod and Atlantic herring are important commercial species and the populations in Skagerrak, Kattegatt and eastern North Sea are examined annually in environmental monitoring. Both species are overexploited by large-scale trawling and together with climate change, the populations have become stressed and have decreased in numbers. All teleost fish have otoliths (earstones) which are commonly used as a tool to determine the diet of marine mammals, birds and predatory fish. What determines the correlation between fish size and otolith size is not fully known, but it is thought to vary depending on food availability, climate and metabolism. Previous research has shown that the correlation between otolith size and fish size can vary between locations, populations and temperature. A change in temperature can affect that both otolith size and fish size grow differently. To investigate whether the correlation has changed, otoliths and fish length from published data from 1978-1979 were compared with new data from 2021-2022. The otoliths from 2021-2022 were photographed and measured through the RStudio software package ShapeR. The correlation between fish size and otolith size was analyzed by an ANCOVA test in RStudio to find out if there was any significant difference between the past and more recent data. The analysis showed that there was no significant difference between fish size and otolith size in cod. This result shows that cod's growth pattern has not changed in this respect, and the correlation can continue to be used in today's diet studies.This also indicates that the new method to estimate otolith length by image analysis is equivalent to manual measurement. For herring, on the other hand, there was a significant difference between fish size and otolith size in the latest data compared to the historical data. Several hypotheses for this difference are discussed, it may reflect altered growth in herring or be linked with different sampling seasons, but it may also be due to different measurement methodology. Therefore, additional studies are needed to assess if there is a true significant difference in relative growth of herring body length and otolith size.