Quiz 7

Harvesting strategies and tactics

References Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26:

Harvest strategies How regulations will change in relation to the state of the fishery State will usually be estimated stock size May also include price, other species, environment Pacific halibut 35% vulnerable biomass Pacific salmon fixed escapement targets Should be robust to environmental changes

Harvest tactics The regulations used to achieve the strategy Time closures Area closures Gear restrictions Vessel numbers Vessel size Pot limits Size limits Bag limits Trip limits Total quota Days at sea

Constant strategies Constant catch Constant harvest rate Constant escapement

Salmon spawners and recruits Adult salmon returning to freshwater from the sea are “total returns” Returns are either caught (catch), or escape (escapement) and spawn Spawners die after producing smolts that head back to the ocean Several years later these return as adults (recruits) in the next generation Generation-to-generation model

Ricker model Replacement line Ricker recruits Surplus production Dashed lines represent MSY conditions: number of spawners, and amount of surplus production (catch) that can be harvested. At MSY: 4600 recruits return, and 2600 of these are caught, leaving escapement of 2000 that spawn, and produce another 4600 recruits in the next generation, etc. If 8000 spawn, these will produce only 3500 recruits. Spawners Recruits 23 Ricker.xlsx, sheet Ricker

Ricker model, with process error Spawners Recruits Spawners Surplus production 23 Ricker.xlsx, sheet RickerProcess

Constant catch policy Catch = 600 per year Catch = 500 per year Generation number Total returns (recruits) Generation number Total returns (recruits) 23 Ricker.xlsx, sheet ConstCatch Escapement Catches

Constant harvest rate Harvest rate = 0.7 Harvest rate = 0.3 Generation number Total returns (recruits) Generation number Total returns (recruits) 23 Ricker.xlsx, sheet ConstHR Escapement Catches

Constant escapement Constant escapement = 4000 Constant escapement = 1000 Generation number Total returns (recruits) Generation number Total returns (recruits) 23 Ricker.xlsx, sheet ConstEsc Escapement Catches

Constant escapement Constant harvest rate Constant catch Average annual catch Average CV of catch Average escapement 23 Const strategies.xlsx

Mean catch vs. mean escapement Average escapement Average catch Constant catch Harvest rate Constant escapement 23 Const strategies.xlsx, sheet comparisons

Mean catch vs. CV of catch Average catch CV of catch Constant catch Harvest rate Constant escapement 23 Const strategies.xlsx, sheet comparisons

Other strategies

Floor-rate strategies Floor = 0, rate = 0.5 Floor = 2000, rate = 0.5 Floor = 5000, rate = 1 Run size Allowed catch 23 Floor rate strategies.xlsx, sheet Description

Mathematical form If C < 0 then set C = 0 Floor Rate Run size Catch

Low catch CV High mean escapement 10010,0005, ,5007, ,0005,0002,5007,500 Floor (minimum escapement) Rate (harvest rate above floor) High mean catch Tradeoff floor vs. rate 23 Floor rate strategies.xlsx, sheet Tradeoff

Periodic harvesting Also known as pulse harvesting Geoducks, clear-cut logging Good if large economies of scale Good if old individuals are particularly valuable and there is no possibility of size/age selective harvesting

Sex-specific harvesting Take the males, they are pretty useless Used primarily in fisheries where animals can be returned to the water with good chance of survival and sex can be determined Crabs, lobsters etc. Caution: Alaskan crab stocks crashed despite males only How to calculate needed sex ratio

Size limits Commonly used in invertebrate fisheries and sport fisheries Set size above age at first reproduction

Walters dilemma We don’t estimate abundance very well, even for trees 20% error is good Fish are like trees except they are invisible and they move Estimates of abundance can easily be off by 100%, the inter-model average is 37% With a 35% harvest rate, if our estimate was double, we would set the quota at a 70% of stock size This is what happened with northern cod Carl Walters

Walters solution Avoid using TACs Close fishing at times and space to ensure there is a realistic maximum harvest rate This has worked for Pacific salmon

Why wouldn’t this work? Such closures would mean an end to the many major fisheries—just what Walters wants to avoid Many fisheries rely on fishing the population when it is most aggregated and thus totally vulnerable But he has a very good point!

Alternative solutions to Walters dilemma Be much more cautious—stay on the right hand side of the yield curve Be much more pro-active and be prepared for rapid changes in quota Accept much higher risk than we normally admit

Walters formula for sustainable harvesting Refugia Economic, e.g. tropical tunas Spatial, e.g. northern cod before offshore trawling Temporal, e.g. salmon Size, e.g. lobsters

Multispecies strategies

Pictures: © Archipelago Marine Research Ltd, © All Enthusiast, Inc. Pacific ocean perchDarkblotched rockfish Widow rockfish Canary rockfish Yelloweye rockfish Bocaccio Lingcod Cowcod Overfished Recovered Tradeoffs in west coast groundfish

Total biomass Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Year Demersal fish biomass (million t)

Overfished stocks only Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Year Demersal fish biomass (million t)

Catch and harvest rate Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Year Total catch (thousands of t) Exploitation rate

Lost yield overfishing vs. underfishing Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Year Lost yield (proportion of max)

Maximizing yield Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Exploitation rate Surplus production (thousands of t)

Maximizing profit Revenue Profit Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Exploitation rate Revenue or profit ($milion)

Catch vs. depletion Collapsed Overfished Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Catch (thousands of tons) Number of stocks

Profit vs. depletion Collapsed Overfished Hilborn R, Stewart IJ, Branch TA & Jensen OP (2012) Defining trade-offs among conservation, profitability, and food security in the California Current bottom-trawl fishery. Conservation Biology 26: Total profit ($million) Number of stocks

Worm B et al. (2009) Rebuilding global fisheries. Science 325: Old target New target Front page, New York Times Exploitation rate Percent of maximum

Tools used to control fishing effort Worm B et al. (2009) Rebuilding global fisheries. Science 325: