Centurial life history parameters in Baltic cod (Gadus morhua)
Data files
Oct 04, 2024 version files 5.16 MB
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Inputdata_figures3_6.csv
9.96 KB
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L50_CF.txt
24.51 KB
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lipid_content.csv
36.14 KB
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README.md
8.26 KB
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Survey_data.txt
5.08 MB
Abstract
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.
README: Centurial life history parameters in Baltic cod (Gadusmorhua)
Authors:
Henrik Svedäng
Baltic Sea Centre, Stockholm University, SE-106 91 Stockholm, Sweden
Sara Hornborg
RISE Research Institutes of Sweden, Department Agriculture and Food, Gothenburg
Anders Grimvall
Swedish Institute for the Marine Environment (SIME), Gothenburg University
*Files found at Dryad. https://doi:10.5061/dryad.4tmpg4fkt *
Survey_data.txt
L50_CF.txt
Inputdata_figures3_6.csv
lipid_content.csv
Tables_supplementary.docx
File descriptions
Survey_data.txt
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). File file consists of extracted data from the archives on Eastern Baltic cod (EBC) used in the above mentioned article. The data from the archives are from three different sources:
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.
The following variables are included:
Data source - 1-3 (see above)
ID_No - identification number
ID_Name - identification name
Area - study area name
Station - specific station
ICES_SD - designated ICES subdivision number
Year - calendar year
Date - date
Month - month
Quarter - quarter of the year
Research_Vessel/Fishery - vessel name
Haul_No - haul number
Nr_Individual - number given to an individual fish
Total_length_cm - total length in cm
Weight_g - total weight in g
Fulton_CF - estimated Fulton’s condition factor
Sex - Fe, Ma, unknown
Maturity - maturity scale, i.e. eight-degree Maier’s scale
Im/mature - estimated immature-1: scale 1-2, mature-2: scale 3-7
Maturation - given as text: immature/ mature
Species - only cod included
Cells with no available data are marked as "NA".
For the latter period between 1991 and 2022, ICES DATRAS database (www.ices.dk) as well as published analytical assessment information (Eero 2021a,b; ICES 2023) have been used.
L50_CF.txt
Estimated L50 at maturity and Fulton’s condition factor by ICES SD (subdivision). The following variables are included:
Year - calendar year
SD - ICES subdivision
SEX - all
L50 - the estimated maturity ogive by ICES subdivision
CF- estimated Fulton’s condition factor by ICES subdivision
CF_no_obs - number of estimated observations in Fulton’s condition factor
Cells with no available data are marked as "NA".
Inputdata_figures3_6.csv
Input data for figures 3 and 6 in the above-mentioned article. The following variables were included:
Year - calendar year
L50 - yearly arithmetic mean value of *L*50 as calculated based on the available annual estimates on *L*50 in SD25-29 (see also Table S3; Tables_supplementary.docx).
cf_sd2529- yearly estimates of Fulton’s condition factor for the size interval 41-60 cm in EBC in SD25-SD29 in the year i by using records on weight (W) and length (L) from the historical surveys between 1937 and 1983, if the number of records >5 (Table S5; see Tables_supplementary.docx)
LDI - length diversity index, LDI, in year *i *was defined by the Shannon-Wiener diversity of the annual length-frequency distribution (Probst et al. 2013, and the references therein) for the annual survey catch where *L*min was set to 20 cm.
Landings_ICES23 - time series between 1946 and 1965 on landings and total catch from 1966 and onwards, including both landings and discards in ICES (2023)
Discard_23 - discards from 1966 and onwards, including both landings and discards
SSB_ICES23 - spawning stock biomass of EBC in tonnes (ICES, 2023))
Recruits_ICES23 - annual recruits of EBC in numbers (ICES, 2023)
TSB>35 - total stock biomass of EBC above 35 cm in total length in tonnes (ICES, 2023)
F_46_ICES23 - Fishing mortality for ages 4-6* *for the period 1991-2020 (ICES 2023)
CBH_SSB_23 - stock spawning biomasses of central Baltic herring in tonnes ICES 2023)
SPR_SSB_23 - stock spawning biomasses of sprat in tonnes (ICES 2023)
meanL95 - length of the 95th percentile
Fat_content - fat content in herring (see lipid_content.csv)
Cells with no available data are marked as "n/a".
lipid_content.csv
The herring samples were regularly collected in October-November at Utlängan at the southeast corner of the County of Blekinge (Bignert et al. 2017). Fat estimates were made on gillnet caught female herring in sizes 17-19 cm. The following variables were included:
stationID – identification of station
station - locality of sampling
year - calendar year
month - month
day - day of the month
Age - age of the sampled fish in years
Fat_percent – estimated fat content as percentage of body weight
Length - fish length in cm
Weight - fish weight in g
Cells with no available data are marked as "n/a".
Tables_supplementary.docx
The following supplementary tables are included in the article.
Table S1a: The number of years of investigation, the total number of hauls, and the number of caught fish per ICES subdivision (SD).
Table S1b: The number of hauls per ICES subdivision (SD) and year.
Table S2. The number of immature (Im) and maturing (Mat) per year, sex and ICES subdivision.
Table S3. The length maturity ogive modelling results (*L*50, and the length percentile 2.5 and 97.5) for fish caught between April and August by ICES subdivision (SD) 24-31 and year. R-square, number of observations and the length interval for each modelling are show.
Table S4. Summary of linear regression of log(W) ~ log(a) + b*log(L) for all individual cod between 20 and 100 cm.
Table S5. The mean Fulton’s condition factor (C.F.) and number of observations within the size interval 41-60 cm by ICES subdivision (SD) 24-31 and year.
Code/software
Logistic regression was fitted to the material, using the r-script “FSA” developed by Ogle et al. (2018) for estimating *L*50 for sexes combined
References:
Bignert, A., Danielsson, S., Ek, C., Faxneld, S., Nyberg, E., 2017. Comments Concerning the National Swedish Contaminant Monitoring Programme in Marine Biota, 2017 (2016 years data). Naturhistoriska riksmuseet, rapport 10:2017.
Eero M (2021a) Indicator for Eastern Baltic cod length structure. Technical University of Denmark Dataset https://doi.org/10.11583/DTU.16887601.v1
Eero M (2021b) Eastern Baltic cod size at maturity and body condition. Technical University of Denmark Dataset https://doi.org/10.11583/DTU.16895254.v1ICES (2023) Baltic Fisheries Assessment Working Group (WGBFAS), 2023, ICES Headquarters, Copenhagen, Denmark. www.ices.dk
Ogle D, Wheeler P, Dinno A (2018) Simple Fisheries Stock Assessment Methods. Package “FSA” https://github.com/droglenc/FSA
Probst WN, Stelzenmüller V, Kraus G (2013) A simulation-approach to assess the size structure of commercially exploited fish populations within the European Marine Strategy Framework Directive. Ecol Indic 24:621-632 https://doi.org/10.1016/j.ecolind.2012.08.026
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).