Aquatic food production systems practice questions.

A fisheries agency sets an annual quota equal to the maximum sustainable yield of a stock. That quota represents

• the total biomass of the stock at the start of the season
• the largest catch the stock can replace each year
• the largest catch the fleet is technically able to land in a season
• the catch at which the stock falls below its critical size

Answer: the largest catch the stock can replace each year

Maximum sustainable yield is the largest harvest that still leaves the population able to regenerate, so it is an output matched to the rate of natural increase rather than to fleet capacity. Treating MSY as what the boats can catch is the commonest error.

Cold-water upwelling zones such as the Humboldt Current off Peru support unusually large fisheries mainly because

• the cold water dissolves far more carbon dioxide than warm water does
• upwelling raises the water temperature and so speeds up fish growth
• nutrients are transferred up into the sunlit surface layer
• deep water carries a much higher concentration of dissolved nitrogen gas

Answer: nutrients are transferred up into the sunlit surface layer

Upwelling is a transfer of nitrate and phosphate from the deep storage into the photic zone, where light also allows photosynthesis, so primary productivity is high and supports a short, productive food chain.

Increasing the legal minimum mesh size of a trawl net is intended mainly to

• reduce the fuel burned by the vessel during each tow
• cut the number of vessels licensed to fish the stock
• prevent the capture of seabirds attracted to the gear
• let juvenile fish escape and breed at least once

Answer: let juvenile fish escape and breed at least once

Larger meshes retain only fish above a size at which most individuals have already spawned, protecting recruitment to the stock. Limiting the number of vessels is effort control, a separate management tool.

During a strong El Nino event the anchoveta catch off the coast of Peru falls sharply. The immediate cause is that

• heavy rainfall dilutes the salinity beyond the tolerance of the fish
• warm surface water suppresses the upwelling of nutrients
• storms keep the fishing fleet in harbour for several weeks
• the fish migrate southwards into colder Antarctic feeding grounds

Answer: warm surface water suppresses the upwelling of nutrients

A layer of warm, nutrient-poor surface water blocks the upward transfer of nitrate and phosphate, primary productivity falls and the food chain supporting the anchoveta collapses. It is a change in the nutrient flow, not a change in fishing effort.

In one fishery the catch per unit of fishing effort stays roughly constant over a decade, during which the fleet adopts sonar, satellite navigation and larger nets. The most likely interpretation is that

• the stock has grown in proportion to the fishing effort now applied
• the stock is stable and the new technology has changed nothing
• the stock has recovered because the fleet is now better managed
• the stock has declined and better technology has masked the fall

Answer: the stock has declined and better technology has masked the fall

Catch per unit effort is only a valid index of abundance if a unit of effort means the same thing over time. When fishing power rises, a constant index means the fleet is working harder in real terms to land the same fish, which is a declining stock reported as a stable one.

Coastal mangrove is cleared to build ponds for farmed shrimp. Which consequence follows most directly from the loss of the mangrove itself?

• The coast loses wave protection and a nursery for young fish
• The ponds require more feed per kilogram of shrimp produced
• Rainfall over the coastal plain falls in the following decade
• The salinity of the open sea offshore rises measurably

Answer: The coast loses wave protection and a nursery for young fish

Mangrove roots dissipate wave energy, trap sediment and shelter juvenile fish and crustaceans, so removing them costs the coast a storm defence and the offshore fishery its nursery. The pond's own feed and effluent problems are real but are a consequence of the farming system, not of the clearance.