51 Ecosystem Overfishing

Description: Ecosystem Overfishing Indices (Primary Production Required, Fogarty, Ryther)

Found in: State of the Ecosystem - Gulf of Maine & Georges Bank (2021+), State of the Ecosystem - Mid-Atlantic (2021+)

Indicator category: Database pull with analysis; Published methods

Contributor(s): Michael Fogarty, Andrew Beet

Data steward: Andrew Beet,

Point of contact: Andrew Beet,

Public availability statement: Source data is not publicly availabe due to PII restrictions.

51.1 Methods

We use the definition of ecosystem overfishing from (JS Link and Watson 2019):

  1. The sum of catches is flat or declining
  2. Total catch per unit effort is declining
  3. Total landings relative to ecosystem production exceeds suitable limits

All of the indices are based on the principle of energy transfer up the foodweb from primary producers.

51.1.1 Fogarty & Ryther Indices

The Fogarty index is defined as ratio of total catches to total primary productivity in an ecosystem (JS Link and Watson 2019). The units are parts per thousand.

The Ryther index is defined as total catch per unit area in the ecosystem (JS Link and Watson 2019). The units are mt km^-2 year^-1

A modification of the indices is used. Total landings are used in lieu of total catch. This will have the effect of reducing the value of the index (compared to using total catch).

51.1.2 Primary Production Required (PPR)

The index is a measure of the impact of fishing on the base of the foodweb. The amount of potential yield we can expect from a marine ecosystem depends on the amount of production entering at the base of the food web, primarily in the form of phytoplankton; the pathways this energy follows to reach harvested species; the efficiency of transfer of energy at each step in the food web; and the fraction of this production that is removed by the fisheries. Species such as scallops and clams primarily feed directly on larger phytoplankton species and therefore require only one step in the transfer of energy. The loss of energy at each step can exceed 80-90%. For many fish species, as many as 2-4 steps may be necessary. Given the trophic level and the efficiency of energy transfer of the species in the ecosystem the amount phytoplankton production required (PPR) to account for the observed catch can be estimated.

The index for Primary Production Required (PPR) was adapted from (D. Pauly and Christensen 1995b).

\[PPR_t = \sum_{i=1}^{n_t} \left(\frac{landings_{t,i}}{9}\right) \left(\frac{1}{TE}\right)^{TL_i-1}\]

where \(n_t\) = number of species in time \(t\), \(landings_{t,i}\) = landings of species \(i\) in time \(t\), \(TL_i\) is the trophic level of species \(i\), \(TE\) = Trophic efficiency. The PPR estimate assumes a 9:1 ratio for the conversion of wet weight to carbon and a 15% transfer efficiency per trophic level, (\(TE\) = 0.15)

The index is presented as a percentage of estimated primary production (PP) available over the geographic region of interest, termed an Ecological Production Unit (EPU). The scaled index is estimated by dividing the PPR index in year \(t\) by the estimated primary production in time \(t\).

\[scaledPPR_t = \frac{PPR_t}{PP_t}\]

The species selected in each year were determined by their cumulative contribution to total landings. A threshold of at least 80% of the total landings is used.

51.1.2.1 Data sources

Data for this index come from a variety of sources. The landings data come from the Commercial Fishery Database (CFDBS), species trophic level information come from fishbase and sealifebase, and primary production estimates are derived from satellites. Some of these data are typically not available to the public.

51.1.2.2 Data extraction

Landings are extracted from the commercial fisheries database (CFDBS) using the methods described in the chapter Commercial Landings Data.

Trophic level information for each species is obtained from fishbase and sealifebase using the R package rfishbase (Froese and Pauly 2019) in tandem with the package eofindices.

Primary Production is estimated using the methods described in the chapter Chlorophyll a and Primary Production.

51.1.2.3 Data analysis

51.1.2.3.1 Primary Production Required

Annual (wet weight) landings are calculated for each species for a given EPU. For each year the landings are sorted in descending order by species and the cumulative landings are calculated. The species that accounted for the top 80% of total cumulative landings are selected. The trophic level for each of these species are then obtained from fishbase/sealifebase. At this point the PPR index is calculated. The units of the index are \(gCyear^{-1}\) for the EPU. The index is converted to \(gCm^{-2}year^{-1}\) by dividing by the area (in \(m^2\)) of the EPU.

To normalize the index the total Primary Production for the given EPU is required. This is calculated as described in the chapter Chlorophyll a and Primary Production. The units are also converted to \(gCm^{-2}year^{-1}\).

The index is then normalized by dividing the index in year t by the total primary production in time \(t\).

51.1.2.3.2 Fogarty and Ryther Indices

Total annual (wet weight) landings are calculated for a given EPU (summed over all species). The units for both primary production and landings are in \(mt km^{-2} year^1\). A factor of (1/9) is used to convert landings to weight in carbon. The area in \(km^2\) of each EPU is obtained from the shapefile used to define the area.

catalog link https://noaa-edab.github.io/catalog/ppr.html

References

Froese, R, and D Pauly, eds. 2019. Fishbase (version 08/2019).
Link, JS, and RA Watson. 2019. Global ecosystem overfishing: Clear delineation within real limits to production. Science Advances 5:6. https://doi.org/10.1126/sciadv.aav0474.
———. 1995b. Primary production required to sustain global fisheries. Nature 374: 255–57. https://doi.org/https://doi.org/10.1038/374255a0.