Centurial life history parameters in Baltic cod (Gadus morhua)

The status of Eastern Baltic cod (EBC) Gadus morhua has remained poor despite low fishing mortality for over a decade, including a fishing ban since 2019. Although the decline in productivity can be explained by lower individual growth and survival rates, other aspects of life history changes such as maturation patterns for EBC has so far not been sufficiently explored. According to current stock assessments, the median size at maturity (L50) has halved from 40 to around 20 cm in total length since the 1990s, while the overall size distribution has become increasingly truncated. It has previously been suggested that changes in L50 can be attributed to both fishing-induced evolution and phenotypic plasticity induced by growth rates. However, since L50 is currently occurring around 20 cm, the maturation process must have been initiated at much smaller sizes, i.e. long before the fish could be caught in the dominant trawl fishery at around 35 cm. In this study, we aimed to further investigate what drivers may have led to reduced productivity in EBC by determining variations in size at sexual maturity in longer time series than has been done before (1930s to 1980s) and include prey productivity and quality. We found that L50 declined already in the 1930s and thereafter remained stable at around 40 cm up to the 1990s. On a centurial perspective, L50 has been positively correlated to growth potential (L95), length diversity, total stock biomass, total catch, and yield per recruit, while Fulton's condition factor  was not related to L50. Our results suggest that the links between life history parameters and external drivers are complex, but the present unprecedented early onset of maturity and hence decline in L50 since the 1990s signals a decline in growth potential, which also has hampered the productivity of EBC.

Methods
Archived reports from Swedish historical trawl surveys, which were conducted for various reasons, contain records on Baltic cod between 1919 and 1983. The archived material is permanently located at the Regional State Archives in Gothenburg (https://riksarkivet.se/-goteborg), where it can be requested. The survey data are available from locations in the Baltic proper (i.e. ICES SD25-SD29). The archived material contains three sources of information:

Source 1: Catch records, including information on the date, location, fishing gear, time of fishing, number of fish per length group (usually 1 cm) per species.

Source 2: Records on individual cod, giving information on the date, fishing method, fishing location, species, fish length, and sometimes including details on sex and maturity stage.

Source 3: Notes written on the envelopes used for storing biological material such as otoliths and scales saved for age reading purposes, providing information on length, location, and date of catch, as well as occasional information on weight, sex, and maturity stage.

These sources are noted in the database. For the latter period between 1991 and 2022 given in the input files, ICES DATRAS database (www.ices.dk) as well as published analytical assessment information (ICES 2023).