Indicators to Monitor
Gathering information about reproductive health and status (e.g., weighing gonads) can help researchers better predict when spawning might occur. Photo © Soames Summerhays
Once a spawning aggregation has been identified, and a baseline of information has been collected (i.e., seasonality, abundance, spatial usage), the next step is to monitor for changes. Objectively measuring success is the only means MPA managers have of knowing if their management strategies are achieving the desired results. To measure success, it is important to link the measures to specific results.
Types of Indicators
The most commonly used indicators are either biological or socioeconomic in nature. In the case of FSAs, biological indictors are used to measure changes in fish populations, whereas socioeconomic indicators are used to measure changes in the well-being of the people who are dependent on fish populations.
Indicators may be fishery-dependent, meaning that the data used to calculate the metrics are taken directly from the fishing industry, or fishery-independent.
Generally, fishery-dependent indicators are much more inexpensive to acquire, because the fishers carry out the sampling themselves in their fishing activities. Data on catches, landings, and sales may already exist for the fishery, and these can be used to develop indicators. Fishery-independent data must be collected independently of fishing activities, and therefore may require extra expense. Some of the more commonly used indicators are listed in the table below.
| Type | Biological Indicators | Socioeconomic Indicators |
| Fishery dependent | Catch per unit effort (CPUE) Fisheries landings by species Trends in fish abundance at the FSA over time Size, frequency or age class distribution of fishes caught from aggregations |
Ability of local fishers to transition to other professions with equal or better compensation and opportunities Level of community support for conservation actions Changes in living standards of community members dependent on FSAs |
| Fishery independent | Numbers of fishes in the FSA Size frequency or age class distribution of aggregating fishes Rate of recruitment to the reefs in the known settlement areas |
Catch Per Unit Effort as an Indicator of Abundance
Catch per unit effort, or CPUE, is commonly used as an estimator of fish abundance. It is usually assumed that CPUE is directly proportional to abundance. For example, if a fisher in 2000 could catch 4 fish per hour, but the same fisher in 2005, using the same gear and technique, could catch 2 fish per hour, it would be assumed that the abundance of fish in that area had decreased 50 percent from 2000 to 2005. CPUE is commonly used because it is a fishery-dependent indicator. In a few cases, the necessary data already exist, especially if local fisheries agencies require fishers to fill out logs, or record their landings. The CPUE index allows managers to look at general trends in fish catch (and possibly abundance) over time.
Relationships between catch per unit of effort (CPUE) and abundance under hyperstability when fish or fisher behavior results in elevated CPUE even as fish abundance declines until the stock starts to collapse, and hyperdepletion when catches fall disproportionately with effort. Based on Hilborn and Walters, 1992. From Sadovy and Domeier 2005
Hyperstability
CPUE does not always work as a reliable indicator, because it may not always be directly proportional to abundance. When CPUE is not proportional to abundance, two situations may occur: hyperstability and hyperdepletion.
Hyperdepletion refers to the situation where CPUE is actually underestimating abundance, i.e., fish abundance is actually higher than the indicator suggests. Hyperstability is a more dangerous situation that often occurs in aggregating fishes. In this situation, the CPUE index remains high while fish abundance is actually low, i.e., fish abundance is estimated to be higher than it actually is. Hyperstability can occur for a number of reasons: it can be due to the inherent schooling behavior present in many fish species; due to fishers’ skill in picking out areas of high fish abundance; or due to fish response towards bait.
As a fish population declines, fish continue to aggregate or school and therefore their local population density remains constant. Although the number of schools or aggregations present may be declining, fishers will continue to fish these high-density schools, and despite a population decline CPUE remains high. Aggregation fishers often use the argument that there are plenty of fish because CPUE remains stable. To convince fishers otherwise will require independent investigations of abundance that verify the actual decline.