Fisheries science
Fisheries management is all about modelling (predicting) the likely fish species of distinct size/age classes
Critical factor is the .

Recruitment – number of fish reaching a certain or - when they enter the fishery.

Stock assessment
Fundamental basics – level of fishing must be balanced by and
If max mortality level is exceeded, stock will , and capacity to will be reduced

Important terms:
Recruit: A fish that from egg until reproductive (or legal fishery) age

: numbers of fish (or value) produced by the recruits and available to the fishery

Collapse – point where a fished population or stock cannot . May be arbitrarily set at of pre-fished biomass (or 50% MSY)

– part of a species population that is reproductively isolated and managed as a single unit

Fishery at MSY
In theory, fishing at MSY is desirable for all:

Keeps fishing level and stock size relatively high – approx. of maximum stock size

Management aim is to regulate fishing to keep population size at MSY

Summary MSY and stock assessment
Typically fishing at MSY keeps the biomass lower (c. 50-75% lower) than the unfished biomass –

Fishing to MSY is risky – if the changes it can be easy to MSY

MSY usually estimated by species – but species interact: approach may be better…

Growth in fish supply outpaces (3.2% per year vs 1.6% per year)

Mostly met by aquaculture, mostly in

Global marine capture c. 80 million tonnes:
10% species

Expansion in NW and WC Pacific, Indian Oceans, contraction in Atlantic and Mediterranean

people engaged in capture fisheries worldwide

aquaculture

growth per year: 94 million tonnes in 2012, of which 66 Million tonnes is food fish, 23 Million tonnes aquatic algae.

China c. of global production

20 million people are engaged in aquaculture (>90% in China)

social

Fish capture and production large source of for women (especially in developing countries)

More than 15% primary capture employment women and up to 90% processing employment

Fishing and aquaculture assure livelihoods of c. of world’s population

Trends – doom and gloom?

Collapsed species
Fishing down food chain
Fishing deeper
Loss
PP utilized

Types of fishery data:

Research survey - – direct measure of biomass for all species collected in the survey. Limited , few comparative datasets

- Estimate of biomass for specific populations of species based on survey and catch effort data – only data on few targeted species

Catch data - Reported biomass of . Global in scope, but does not reflect and IUU, and is influenced by accidental and deliberate , changes in regulation and markets.

Catch data do not track biomass because other things can influence catch (e.g. changes in , market prices for specific species, prices, vessel exclusions, population ).

Also, in a random stationary time series, maximum catch is likely to with a longer time series (so more chance of having collapse figures over time)

Summary so far

Estimating how many fish have been caught is difficult

Catch and effort data are biased in complex ways as these reflect and as well as ecology/biology

Satellite and big data approaches are changing the way we understand, measure and police global fishing efforts

Fishing effects
Direct removal of target species
Changes in structure of target populations
Alterations to populations of fish and benthos
Alterations to physical environment
Food chain effects (trophic , altered pressure)

Fishing down (through) the food chain
Pauly et al (1998). Fishing Down Marine Food Webs. Science 279: 860-863.
Most food fish were trophic levels 4.0-4.5
Argument that we are now moving our predation from 4 to 3 (fishing down).
Also by removing predators, trophic level biomass expands, decreasing mean TL (fishing through)

Alternative : Catch TL is not the same as ecosystem TL
Theoretical comparison based on simulation ecosystem (week 3)

Catch TL poor reflection of ecosystem TL when catch is within the food web

Recent data from research trawls are suggesting that deep-sea fish species (including by-catch) biomass is .

Summary: state of world fish

Globally, best estimate is that c. of fish stocks are over fished, are fully exploited, and 10% are underexploited

Aquaculture provides fish than capture fisheries

Long history of management failure, stock collapse.

Current ecosystems have low and with body sizes. Top predators heavily

Catch data easily available but may give picture of ecosystem state and trends

Rebuilding occurring in heavily regulated fisheries

vessel monitoring and big data processing give us a much richer view of fishing effort

More to be done in developing world fisheries