Fisheries Management: Effort, Selectivity, and Regulation

Posted on Dec 6, 2024 in Electronics

Fishing Effort

Fishing effort is the activity performed in a fishery, determined by the fishing power of each vessel and the time spent fishing. Total effort is the sum of fishing effort across all vessels and their respective fishing times.

Total fishing effort depends on: number of boats, individual boat effort, fishing power, and fishing time.

To optimize fishing, any of these variables can be adjusted. Gear selectivity separates the population into groups. Selectivity influences the catch composition.

Types of Selectivity

Two main types of selectivity are recognized:

  1. Separation into two groups (e.g., trawls, purse seines).
  2. Separation into three groups (e.g., longlines, handlines, gillnets).

Selectivity into Two Groups

Larger individuals are typically retained, while smaller individuals escape, allowing younger classes to survive.

Selectivity Curves

Selectivity is the probability of being retained, expressed as a function of fish size. The 50% selection point (TS) is the size at which fish have an equal probability of being caught or escaping.

Factor selection (FS) is the ratio between the selection size and mesh opening or length: FS = Selection Size / Mesh Length.

Example: TS1 (147mm) / Mesh1 (40mm) = 3.67; TS2 (215mm) / Mesh2 (60mm) = 3.58; TS3 (290mm) / Mesh3 (80mm) = 3.62. Average FS = (3.67 + 3.58 + 3.62) / 3 = 3.6.

If the legal size is 25cm (250mm), then Mesh Length = Selection Size / FS = 250mm / 3.6 = 69mm.

Selectivity into Three Groups

For longlines, handlines, and gillnets, very small fish are eliminated by hook size, very large fish may not be captured, and a middle group is retained proportionally. The 50% retention size (TS) is also used here.

Methods for Determining Gear Selectivity

Covered Cod-End Method

A smaller mesh net is wrapped around the cod-end to collect escaping fish. Comparing the sizes of fish retained in the cod-end and the cover net allows for the creation of a selectivity curve.

Total Fishing Effort

Total fishing effort depends on: number of boats, individual boat effort, time spent, fishing power, boat tonnage, detection systems, and gear.

Catch Per Unit Effort (CPUE)

CPUE is the relationship between total catch and effort (or number of vessels). Higher effort may lead to lower CPUE. Optimizing effort maximizes CPUE.

Methods Affecting Size or Age at First Capture

  1. Minimum Size Establishment: Prevents capture and destruction of undersized fish.
  2. Closed Areas: Protects juvenile fish in concentrated areas.
  3. Juvenile Catch Limits: Restricts the proportion of juveniles in the catch (e.g., Norwegian Arctic cod).
  4. Closed Seasons: Regulates fishing effort by time restrictions.
  5. Gear Selectivity Modification: Adjusts mesh size or hook size. In multispecies fisheries, regulations may only be effective for one species. Not suitable for purse seines, where large meshes can act as gillnets.

Methods Affecting Fishing Effort

  1. Fleet Growth Limitation: Effective for long-term effort regulation.
  2. Restricted Access: Limits access to fishing grounds.
  3. Fishing Power Limitation: Controls vessel efficiency (hold size, engine power, detection systems) or gear effectiveness (gear bans, size limits, hook limits). Effective for short-term effort regulation.
  4. Area Closures: Permanent or temporary closures, suitable for ecologically mature species.
  5. Time Restrictions:
    1. Fishing bans or closures (biological unemployment), suitable for species with seasonal catchability.
    2. Limits on fishing hours per day, week, or month.
  6. Catch Limits: Total Allowable Catch (TAC) or annual quotas. Requires extensive data collection for accurate assessments.