Ecosystem Overfishing Indicators

Ryther index; total landings presented on a unit area basis for the MAB. Theoretical estimates (Link and Watson, 2019) imply the index should range from 0.3 - 1.1 mt per sq km annually (green area) with a limit of 3 mt per sq km annually, above which tipping points could occur in fished ecosystems (orange area). Expected system-wide MSYs can be in the range of 1 to 3 mt per sq km (unshaded).

Ongoing analysis

Baseline primary production

• Calculated regional estimates of C using similar approach (average TL, TE)
  • PP for MAB from satellite data (data source)

Use species trophic level information

• Contrast with species specific trophic level information (from fishbase)

• Calculate a system wide "average" trophic level, $TL_{a}$, using the idea that primary production required to support the total catch is equal to the sum of primary production required for each species catch

  $$\sum_{i=1}^n C_i \left(\frac{1}{TE}\right)^{TL_i -1} = \left(\sum_{i=1}^n C_i \right)\frac{1}{TE}^{TL_{a}-1}$$

Proposed Analysis

Ecosystem WG Discussion

• Metrics to evaluate simulated ecosystem overfishing

  • Single species overfishing and overfished
  • Metrics applied in other studies (Fulton, ecosystem)
  • Metrics used in NEFMC pMSE
  • Metrics of system variability, Nyquist stability criterion
  • Evaluate impacts to species, functinal groups, fisheries, markets

• If possible, evaluate the status of the ecosystem that implements current SSC ABC recommendations across all species (and other management bodies)

• Tie in with EAFM Risk Assessment

EAFM Risk Assessment Review

State of the Ecosystem → MAFMC Risk assessent example: Commercial revenue

This element is applied at the ecosystem level. Revenue serves as a proxy for commercial profits.

Risk element: CommRev, unchanged

SOE Implications: Recent change driven by benthos. Monitor changes in climate and landings drivers:

• Climate risk element: Surfclams and ocean quahogs are sensitive to ocean warming and acidification.
• pH in surfclam summer habitat is approaching, but not yet at, pH affecting surfclam growth

New elements under discussion

Next SSC update in March 2024!

Appendix: Ecosystem Overfishing Indicators

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
• The fraction of this production that is removed by the fisheries

Ryther, Fogarty, Friedland Indices

The Ryther index consists of total catch presented on a per unit-area basis for an ecosystem.

The Fogarty index is the ratio of total catches to total primary productivity in an ecosystem.

The Friedland index is the ratio of total catches to mean annual chlorophyll in an ecosystem.

From (Link, 2021) "One can develop and estimate thresholds for them based on first principles, trophic transfer theory, and empirical evidence." Thresholds were proposed (Link, et al., 2019)

Link and Watson 2019 overview

Link and Watson 2019, https://www.science.org/doi/10.1126/sciadv.aav0474

All three indices are based on the principle of energy transfer up the foodweb from primary producers. There are limits to how much an ecosystem can produce and potentially be caught, C in tons $year^{-1}$

$$C = \alpha PP.TE^{TL-1}$$ where

$PP$ = primary production ( tons C $year^{-1}$ )

$TE$ = Transfer efficiency

$TL$ = Trophic level

$\alpha$ is "a scalar for local conditions (set to 15% -20% for average availability of the TL in estimating total catch, a value that emerges when estimated from maintaining a global average catch that has been stable for the past 30 yrs)"

This equation is primary production required (PPR) rearranged to express Catch in terms of PP rather than PP in terms of Catch (at the system aggregate level)

Link and Watson 2019 overview

Link and Watson 2019, https://www.science.org/doi/10.1126/sciadv.aav0474

Link and Watson 2019 overview: Ryther Index

Link and Watson 2019, https://www.science.org/doi/10.1126/sciadv.aav0474