20 Fish Condition Indicator
Description: Relative condition
Found in: State of the Ecosystem - Gulf of Maine & Georges Bank (2018+), State of the Ecosystem - Mid-Atlantic (2018+)
Indicator category: Database pull with analysis
Contributor(s): Laurel Smith
Data steward: Laurel Smith, laurel.smith@noaa.gov
Point of contact: Laurel Smith, laurel.smith@noaa.gov
Public availability statement: NEFSC survey data used in these analyses are available upon request (see BTS metadata for access procedures). Derived condition data are available here.
20.1 Methods
Relative condition (Kn) was introduced by Le Cren (1951) as a way to remove the influence of length on condition, and Blackwell, Brown, and Willis (2000) noted that Kn may be useful in detecting prolonged physical stress on a fish populations. Relative condition is calculated as \[Kn = W/W',\] where \(W\) is the weight of an individual fish and \(W'\) is the predicted length-specific mean weight for the fish population in a given region. Here, relative condition was calculated for finfish stocks commonly caught on the Northeast Fisheries Science Center’s (NEFSC) autumn bottom trawl survey, from 1992-present.
For this work, length-weight coefficients from Susan E. Wigley, McBride, and McHugh (2003) were used to calculate W’. Individual fish weights were total body weights from Northeast Fisheries Science Center (NEFSC) fall bottom trawl surveys. Most finfish species included in this study are spring or summer spawners, so the fall survey was chosen to reduce variability of gonad weights in the spring survey as butterfish ramp up for spawning. Kn was averaged on a NEFSC bottom trawl strata resolution.
The Condition package used for calculations and plotting of fish condition factor can be found on GitHub.
20.1.1 Data sources
Individual fish lengths (to the nearest 0.5 cm) and weights (grams) were collected on the NEFSC bottom trawl surveys from 1992-present aboard RVs Albatross IV, Delaware II and the Henry B. Bigelow (see Survdat). A small number of outlier values were removed when calculating the length-weight parameters.
20.1.2 Data extraction
Data were extracted from NEFSC’s survey database (SVDBS) using the R script found here
20.1.3 Data analysis
Relative condition is calculated by fish species and EPU as (\(Kn\) formula found above) where \(W\) is the weight of an individual fish and \(W'\) is the predicted length-specific mean weight for the fish population in a given region. Predicted weight was calculated as:
\[\textrm{Weight} = e^{Fall_{coef}} * \textrm{Length}^{Fall_{exp}},\]
where \(Fall coef\) and \(Fall exp\) are from Susan E. Wigley, McBride, and McHugh (2003).
The code found here was used in the analysis of fish condition.
catalog link https://noaa-edab.github.io/catalog/condition.html