Multidecadal changes in fish growth rates estimated from tagging data: A case study from the Eastern Baltic cod (Gadus morhua, Gadidae)

Abstract

Long time series of reliable individual growth estimates are crucial for understanding the status of a fish stock and deciding upon appropriate management. Tagging data provide valuable information about fish growth, and are especially useful when age‐based growth estimates and stock assessments are compromised by age‐determination uncertainties. However, in the literature there is a lack of studies assessing possible changes in growth over time using tagging data. Here, data from tagging experiments performed in the Baltic Sea between 1971 and 2019 were added to those previously analysed for 1955–1970 to build the most extensive tagging dataset available for Eastern Baltic cod (Gadus morhua, Gadidae), a threatened stock with severe age‐determination problems. Two length‐based methods, the GROTAG model (based on the von Bertalanffy growth function) and a Generalized Additive Model, were used to assess for the first time the potential long‐term changes in cod growth using age‐independent data. Both methods showed strong changes in growth with an increase until the end of the 1980s (8.6–10.6 cm/year for a 40 cm cod depending on the model) followed by a sharp decline. This study also revealed that the current growth of cod is the lowest observed in the past 7 decades (4.3–5.1 cm/year for a 40 cm cod depending on the model), indicating very low productivity. This study provides the first example of the use of tagging data to estimate multidecadal changes in growth rates in wild fish. This methodology can also be applied to other species, especially in those cases where severe age‐determination problems exist.

Temperature impacts on fish physiology and resource abundance lead to faster growth but smaller fish sizes and yields under warming

“…S4). The cut-off at 2002 also ensured that we did not calibrate the model to the period starting from mid 2000's when the growth capacity, condition, proportion of large fish in the population, and reproductive capacity of cod started to decline rapidly (Svedäng & Hornborg 2014;Casini et al 2016;Mion et al 2018Mion et al , 2021Neuenfeldt et al 2020).…”

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

“…The cod stocks also differ in several other parameters, e.g. the WBC stock has a lower stock size [ 11 ] but cod in the western Baltic Sea have a higher growth rate and reach larger sizes [ 12 , 13 ], their livers are less infested with anisakid larvae [ 14 ] and they have more easily readable otoliths [ 15 ].…”

Temperature impacts on fish physiology and resource abundance lead to faster growth but smaller fish sizes and yields under warming

“…We chose to calibrate our model to the time period of 1992–2002 as in Jacobsen et al ( 2017 ), which is a period after an ecological regime shift, characterized by high fishing mortality on cod, low cod and herring abundance and high sprat abundance (Gårdmark et al, 2015 ; Figure S4 ). The cut‐off at 2002 also ensured that we did not calibrate the model to the period starting from mid 2000's when the growth capacity, condition, proportion of large fish in the population and reproductive capacity of cod started to decline rapidly (Casini et al, 2016 ; Mion et al, 2018 , 2021 ; Neuenfeldt et al, 2020 ; Svedäng & Hornborg, 2014 ).…”