Raw catch data should generally not be used as an indicator of stock depletion. This is because many factors besides in-the-water abundance can affect catch (e.g., weather, gear changes, fish behavior, etc.), and because without an understanding of effort (e.g., How many people are fishing at a time? Where and when are they fishing? Are they targeting the stock when it aggregates? What methods are they using to locate and track the fish? Etc.) there may be significant decoupling of catch trends from abundance trends. For example, an increase in catch over time could mean that fish abundance has been steadily increasing, or it could mean that there were a few strong year classes in the recent past that are being fished down, or it could mean that the entire population is being fished down from initial high abundance levels as fishery effort increases, or that prices have increased and therefore fishermen are catching more irrespective of fish abundance. For certain kinds of fisheries (e.g., those targeting spawning aggregations), catch trends provide little useful information regarding stock status until stock abundance declines to quite low levels that can affect catches on the aggregation.
In addition, the use of catch data alone may inaccurately represent abundance trends and status if the catch trend is not reflective of the entire history of the fishery. For example, catch records from the earliest history of the fishery tend to be low because the capacity of the fishery has not fully developed. Therefore, low historical catch records do not typically reflect low historical abundance levels and are thus a poor proxy for current abundance levels. Similarly, if catch records are not available for a portion of the time series (i.e., a set of years where fishers were not reporting their catch) this can lead to uncertainty in trends. Catches can be high when abundance is high, of course, but catch can also be high when abundance is low if effort (e.g., the number of fishers) has increased dramatically. This further reduces the utility of catch trends as a proxy for abundance. Moreover, using catch and other historical data to evaluate current stock status or estimate reference points is risky because ocean conditions change and are changing even more rapidly and dramatically as a result of climate change. Abundance levels of the past simply may not be possible today.
Catch per Unit Effort (CPUE) is generally an improvement on raw catch data because it accounts for changes in effort. Trends in CPUE, either among season or within season, can be used directly as an indicator of fishery performance without any further analysis. However, it’s preferable to use statistical methods to correct or standardize ‘raw’ CPUE data for external effects such as seasonality, use of gears that target fish (e.g., light sticks on longlines), or other factors that change the catchability: the relationship between catch rates and abundance (see Campbell 2004 in Resources). Length composition data and/or fishery independent biomass surveys can also help you interpret CPUE trends.
CPUE can be a useful indicator of profitability, because when catch is high and effort is low (resulting in high CPUE), revenues from the sale of the catch can be high while fishing costs can be low because low levels of effort (which entails fuel, labor, and other costs) are expended to catch the fish. Hence profits can be high. CPUE trend is often also used as an indicator of stock abundance, since it seems logical that CPUE would be high when stock abundance is high, and that CPUE would be low when stock abundance is low – less fish in the water means that they are harder to find and catch, resulting in lower catch and higher effort (lower CPUE). However, it is important to critically examine other factors that could explain CPUE trends before concluding that stock abundance is low or high based on CPUE. For example, if fishermen have improved their ability to target patches of fish that remain in a declining population, CPUE could be high or stable even if stock abundance is low or decreasing. For this reason, it’s best to use other indicators of fish abundance that are more direct measures, such as fish density estimated from scientific (random) fishing surveys or visual surveys.
Inputs:
- Catch-Per-Unit-Effort (CPUE) for more than three years (ideally more than 5 years)
- Length-frequency of the catch for more than three years (ideally more than 5 years)
Outputs:
- CPUE and trends in CPUE
- Proxy for abundance and trends in abundance
- Average length and trends in average length
Input Sensitivities, Assumptions and Caveats:
- This method depends on reliably tracking the total catch and effort
- Assumes that change in CPUE corresponds to change in biomass/ abundance, which can be very difficult to confirm as a result of targeting behavior or the formation of patches of fish.
- CPUE is seldom proportional to abundance over a whole exploitation history and an entire geographic range, because numerous factors affect catch and effort.
- CPUE can remain relatively stable or even increase while stock abundance is declining. Thus, stable or increasing CPUE levels could provide a false sense of security
Reference points:
Reference points for trends in CPUE will depend on the characteristics of the stock and fishery, as well as the values, goals, and risk tolerance levels of stakeholders.
Recommendations:
- CPUE trends can support the interpretation of other analyses, for example of fishing morality of spawning potential ratio (SPR).
- CPUE can also serve as an indicator of economic status, because high CPUE is often related to higher profit levels (fishing costs are lower when CPUE is high)
- Understanding how the trends in CPUE fluctuate from one year to next or in comparison to the historic trends is essential to use catch trends for management.