# Presentation: 514715

## Multispecies Production Models:Past and Present, Examples at the NEFSC

***By***
***W.J. Overholtz, J.S. Link, H. Moustahfid, R.J. Gamble, M.C. Tyrrell, B.E. Smith***
***NEMOW-August 2007***

## 2-Tier Surplus Production Analysis

***Brown et al 1977: analysis of the ICNAF bottom trawl fishery in SA 5+6 during 1961-1972.***
***Mayo et al 1992: re-analysis with additional years from 1973-1987.***
***Aggregate catch and effort data to estimate surplus production and BRPs.***

## Summed Quota 1,300,000 mt

***Brown et al. 1976 Catch and Effort 1961-1972***

***Aggregate MSY 900,000 mt***
***Summed Quota 1,300,000 mt***

## Mayo et al. 1992- Catch and Effort 1960-1987

***Mayo et al. 1992- Catch and Effort 1960-1987***

***1960-1976***

***1977-1987***

***Aggregate MSY 700,000 mt***

## Conclusions

***“...******summing the MSYs from individual assessments may be an overestimate of the total MSY.”***
***The 2******nd****** Tier Quota was 30-45% lower than the sum of the individual quotas (1.3 e6 mt).***
***Similar results from LP analyses: weak stock management necessary!***

## Gulf of Maine-Georges Bank Herring

***Surplus Production Accounting for Predation***

## Atlantic Herring M2 and F during 1959-2002

***Atlantic Herring M2 and F during 1959-2002***

## Long-Term Yield

***Fox  1975 SP model***

## Modified Fox model

***Modified Fox model***

## Surplus Production with Predation Included

***1.8  ***

**1.4**

**1.0**

***Surplus Production with Predation Included***

## If predators increase, less fishery SP would be available

***If predators increase, less fishery SP would be available***

***At Bmsy=1452 kt***

## Predator vs Fishery Tradeoff

***Predator vs Fishery Tradeoff***

**2007 1200**

**2026 1400**

**At Bmsy=1452 kt**

## Conclusions

***Predation mortality (M2) should be included in stock assessments of prey fish.***
***SS assessments of prey fish are generally optimistic relative to BRPs***
***If the fishery and predators utilize the same size spectrum of prey, then tradeoffs are probably warranted.***

## Guild-Based Biomass Production Model

***We examined a suite of ecological and harvest scenarios.***
***The ecological scenarios: modify competition and predation parameters (nsh). ***
***The harvest scenarios: modifying harvest rates.***
*** ******The goal was to investigate general properties and  responses in a Guild System***

## Modified Schaefer Model

***Modified Schaefer Model***

***N =Biomass         B = Competition Coefficient    I = species***
***R = Growth Rate  a = Predation Coefficient        p = predator (on species i)***
***H = Harvest Rate  K = Carrying Capacity            g = Guild Member***
***                                                                                ******G = Guild***

## Ecological Scenarios

## INTER-GUILD COMPETITION 

*** ***
*** ******1. The overall effect was to lower each individual Benthivore’s biomass***
*** ******2. Total Benthivore biomass dropped to second in guild ranking, ***

_***INTER-GUILD COMPETITION ***_

***Inter-Guild Competition was doubled on Benthivores***

## overall ranking within the guild stayed relatively consistent.  

***overall ranking within the guild stayed relatively consistent.  ***
***B3, B4 and B5 crashed.***
*** ******Due to the reduction in Benthivore biomass,***
***3. Shrimp-Amphipods increased since the two guilds interact through Inter-Guild Competition.***
*** ******4. Total Benthivore biomass dropped from first to last in guild ranking.***

_***PREDATION SCENARIO***_

***Predation was doubled on Benthivores.***

## Harvest Scenarios

## DEMERSALS OVERFISHED

***1. total system biomass increased.  ***
***2. Piscivores and Shrimp-Fish declined.***
***3. Planktivores increased and k increased for this guild.***
***4. Benthivores increased due to a higher growth rate, and a reduction in competition and predation.***
***5. Reduction in predation caused an overall biomass increase.***

## PISCIVORES OVERFISHED

_***PISCIVORES OVERFISHED***_

***1. Total biomass of the system increased, ***
***2. Rankings within the Planktivore guild changed.  This was because more predation occurred on PL1 and PL2 than PL4.  ***
***3. When predation effects were lowered, PL1 and PL2 were able to out compete PL4. ***

## Results for this scenario show a strong similarity to the historical situation in the Northeast U,S, Atlantic Ecosystem .  Starting in the early 1990’s, the Piscivore biomass began to decline, resulting in an increase in Planktivore biomass.  Additionally, increasing trends in Benthivore and Shrimp-amphipod biomasses occur in both the real world situation and this scenario.

## PELAGICS OVERFISHED

***1. Total biomass of the system decreased.***
***2. Most of this decrease can be attributed to the much lower biomass of Planktivores***.

_***PELAGICS OVERFISHED***_

## DOMINANT SPECIES 

_***OVERFISHED***_

***1. overall biomass of the system increased due to the reduction of the most important predators in the system.  ***
***2. While some guilds showed a new dominant species under this scenario, after 30 years, B1 again was the dominant species, due primarily to its only predator’s (P1) loss of biomass.***

## CONCLUSIONS

***1. Predation appears to be the dominant biotic term within the model. ***
***2. Overall biomass of a guild tends to be stable. even though individual species biomasses may change greatly.***
*** ******3. Changes in one biotic factor may cause unexpected results due to predatory and competitive interactions.***
***4. A demonstration copy can be obtained by contacting Robert Gamble (Robert.Gamble@noaa.gov).***

## General Conclusions

***Rich History of Past and Current Use.***
***Sum and Parts are Important***
***Addresses the Issue of Biological and Technological Interactions***
***Could this Approach to Simplify Management***.