Download How can stock assessment be improved?

Opportunities for improving
stock assessment
Kristjan Thorarinsson
Population Ecologist
The Federation of Icelandic
Fishing Vessel Owners
Ingredients for good assessment
Required as a minimum:
• Sound knowledge
• Good dynamic and statistical models
• The right info/data
Knowledge, model development and data
collection progress in an interactive way
Assessing scientific advice:
How good is it?
• Sound biological knowledge √
• Expertise in modelling and statistics√
• Full use made of all available data?
A gap in knowledge
• Fisheries data are produced by an interaction
between fish stocks and fishing fleets
• Fisheries scientists make great effort to understand
fish stocks
• Fisheries scientists make relatively little effort to
understand fishing fleets
• Consequently, the interaction between fleets and
fish stocks is poorly understood
Assessment not automatic
Many judgement calls must be made when
assessing any given stock.
(Do I make this assumption or that assumption? Do I
believe the results of this model or that model? What info
can guide me?)
Many sources of information can and
should affect such judgement calls.
Industry expectations
• The fishing industry will always demand
improved performance
• Current post hoc explanations of failure not
adequate. Catchability change? Can happen
again if not fully understood.
Need for improvement
• Points illustrated with 2 Icelandic
examples:
• 1. Cod - a stock assessed only on the basis
of survey
• 2. Saithe - a stock previously estimated only
on the basis of commercial catch data
Recent asessments:
Over and underestimates
MRI track record in recent years has led
to doubts:
• Saithe stock -- underestimated
• Cod stock -- overestimated
MRI needs to rebuild confidence in its
assessments and advice
Saithe: A trap
• Insufficient monitoring of recruitment and
migration
• Most all data come from fishery (catch and CPUE)
• Vicious circle: TAC => avoid saithe => catch
and CPUE => stock estimate => TAC
• No way for TAC to go up? Requires independent
data, either stock survey or crew survey
Cod
• Overestimate => CCR gives too large catch
=> stock decline and serious
disappointment.
• Groundfish survey CPUE has produced
more accurate results than trawler log book
CPUE. Reason: Technical change and
changes in fleet behaviour.
Fleet data
• Fishing operations do not constitute planned
scientific sampling of fish stocks. Nevertheless
there may be a useful (fixed) relationship between
average CPUE and stock abundance if vessel and
gear remain unchanged and if fishing objectives
remain unchanged.
• Info from vessel captains is needed for proper
interpretation of log book data.
Survey data
• Surveys independent of commercial fishing
operations are relatively reliable in some
cases. Survey results often need to be
supplemented with information from the fishery,
especially when area coverage is limited.
Adequate data?
• Stock redistribution. An inshore shift in cod
distribution suspected in 2000-2002.
Annual groundfish survey does not appear
to cover the inshore adequately
=> Need to institute a new inshore survey
Adequate systematic use of data?
Fishing fleet operations:
–
–
–
–
–
–
–
–
technical change
mesh size change
quota transfers
target species
log books
interviews with captains
scientist participation in fishing trips
digital data on individual fish from processing
Systematic study needed
New and Trendy:
Ecosystem Based
Fisheries Management
• Everybody talks about it
• Nobody knows what it is!
• Some steps have already been taken
• Appearing in official texts
• A great deal of work ahead
Conclusion
In fisheries assessment and advice, it is in my opinion not
sufficient to apply population dynamics models to standard
data on and catch and CPUE. It is very important also to
follow the fishery itself in some detail, how the fishing
boats operate, how the fishing effort and fishing techniques
change, and what the fishermen have to say about fish
behaviour and changes in fish availability. This
necessitates organised communication and cooperation
between fishery scientists and the fishing industry. A lot
more can be done profitably in this regard than is current
practice.