NEFSC Trawl Bodymass Distribution Allocations

Supplemental materials for size spectrum manuscript.

Author
Affiliation

Gulf of Maine Research Institute

Published

November 28, 2022

Patterns in Biomass Allocation

The following code digs into where the biomass is allocated among the 62 species sampled by the NE Groundfish Survey. Stratified abundances are used. Individual lengths and bodymasses are used to isolate what proportion of the overall abundances and biomasses reside.

Data is prepared and updated using {targets} to ensure a consistent data state and a reproducible workflow.

Target Data

Data for the report comes directly from the {targets} workflow found in _targets.R. For this markdown I’m loading in the results from the size spectrum slope analysis and the data that went into it so I can dig into any odd patterns I see. There is also regional SST based on the same trawl regions.

Exploration of Abundance/Size Information

The biological data was filtered prior to doing the size spectrum analysis so that any individuals smaller than 1g were removed. Then to dig into where the biomass andd abundances were distributed I’ve made some size bins to help tease out what the community looks like.

Group Summary Functions

The following function is used to process the same numbers for each combination of factor groups without me rewriting the function over and over again.

Same idea here, but for plotting them based on their groupings

All Data

These figures are helpful for just displaying how the overall size distributions have changed with abundance, biomass, or species type over the years:

Regional Differences

These first plots get at how the different regions compare to one another. Are we seeing striking patterns across them all simultaneously, or are there localized patterns that only occur in some of them.

Seasonal Differences

Functional Group Differences

Thought it may be helpful to have a table here to check which species are currently assigned to which group.

Fishery Status Diffferences

Thought it may be helpful to have a table here to check which species are currently assigned to which group.


Community Size Spectra Results

Community size spectrum slopes were estimated using 2 methods for comparison. An individual size distribution approach ISD developed by A. Edwards and using a more traditional method of binning biomass information into bins before fitting a slope to those bins.

The first method uses the {sizeSpectra} package which were shown to be the most accurate when using simulated data compared to any of the binning methods.

The second method uses binned abundances, with bodymass bins of width 0.5 on a log10 scale, so 10^0 - 10^0.5 etc. These bins were then normalized by dividing the abundances by the bin width to account for the increasing bin width.

Starting data used for both methods was the same. A minimum individual biomass of 1 gram was used to avoid issue with sampling biases for smaller individuals. Area-stratified total abundances (and their corresponding length-based biomasses) were used to preserve the importance of the sampling design.

Pull Results from Both Methods

Biomass Data to Match Size Spectrum Analysis

The first lens to look at is the seasonal variation across all the different areas. This was the typical grouping that we have been focusing on.1

The second lens that I feel is important is the functional groups. This is the not the typical grouping that we have been focusing on, but clears the air on where the biomass increase is coming from.

Edwards Methodology

The Edwards methodology differs from the other methods for estimating size spectra by avoiding the subjective decisions around how to bin data prior to fitting the log-linear regression.

Instead, Edwards uses the individual size distributions (how many individuals of any given length/weight). Abundances are totaled into discrete size bins based on expected biomass at length and length + 1, and individuals that fall within those bins are totaled to get abundances across a continuous distribution of individual bodymass.

The next difference is that the individual body size data is presumed to follow a bounded power-law distribution, with a minimum and maximum body size. Using the individual size data, maximum likelihood estimation is used to solve for the parameter (b) that minimizes the negative log-likelihood.

Odd Years

In the previous figure, for Georges Bank there are a number of years when b is close to -1.2 that do not follow the rest of the trend. These occur on 1974 and 1980. This section exists to see why that is:

The individual size distribution plots look like a lot but are not too complicated with a little explanation.

The blue bars are the width of what a given species biomass can be based on. What the species is, and its L-W relationship. Since fishes are measured to the nearest cm the left bound is the biomass for that length, and the right bound is how much mass it could also have before jumping into the next 1cm increment.

The height of the blue bars indicate how many individuals fall within that size range. Eventually building up the range of possible sizes caught, and how many there are/were.

The red line is a bounded power law fit to those blue lines. The power law relationship between abundance and size is the foundation for all the size spectra methods, in this case individual sizes are used rather than the more typical larger bins.

Data Prep

Plotting Function

Individual Size Distributions

Like any of these methods sometimes the assumed relationship does not perfectly fit the data. In the case of these two years there appears to be in the 10g-1kg range.

If I plot the same data using the binning approach you still see that the bins that cover 10^1 through 10^3 grams are higher than the fit line (analogous to red line for ISD). But, because the size distribution is no longer continuous, you can potentially run into issues of bins not accurately representing the data that goes into them.

Log10 Binning Results

For each of the slope/intercepts derived using a linear model and binned data I also pulled the adjusted r-square to get a sense of whether or not certain groups had poor fits that should be investigated.

Sea Surface Temperature

External Driver Timelines

Several external forces have been suggested as both drivers and/or contributors to changes in the composition of the Northeast Shelf fish community.

Among them are: * Changes in temperature regimes
* Historical fishing removal
* Climate Oscillations * Trophic Cascades

Each of these have different time horizons and periodicities associated with them. This timeline aims to place them on a common timeline to see the agreement or alignment between competing factors.

Historical periods related to fishing gear changes and the decline of the groundfish fishery can be found here:A brief History of the Groundfishing Industry of New England

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A work by Adam A. Kemberling

Akemberling@gmri.org

 

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