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Have discarding practices changed?

Indicators: Discards | Bycatch

Short Answer: No. Bycatch of whales, porpoises, and sea turtles generally did not change after implementation of catch shares; those changes that did occur were not clearly attributable to the catch share program.


Key Findings

  • Observed rates of interaction between fishing gear and whales, dolphins, and sea turtles depended greatly on both gear type and vessel size.
  • For most gear types and vessel sizes, there was no discernible change in interactions (per unit effort) coinciding with the implementation of catch shares.
  • For large vessels using otter trawls, there was an increase in common dolphin and pilot whale interactions. Yet these increases were not clearly associated with changes in where or when observed fishing trips occurred.
  • More detailed analysis is needed to determine if the changes in common dolphin and pilot whale interactions were caused by environmental changes that made these populations more vulnerable, or whether they can be linked to changes in fishing activity associated with catch share management.

Interactive Chart Story


This indicator shows the average number of whales, dolphins, and sea turtles that were caught, killed, or harmed per 1,000 hours of fishing activity by vessels using otter trawls or gillnets, when third-party, onboard observers were monitoring bycatch.


Otter trawls and gillnets are the two commonly used gears in the Northeast multispecies fishery. Like other types of large mesh fishing gear, otter trawls and gillnets can incidentally catch, kill, or otherwise harm marine mammals and sea turtles. These species are protected by federal law, and the number of lethal interactions are regulated. Accordingly, fishing crews generally seek to avoid interactions with protected species. 

The following results are based on data reported by fisheries observers and at-sea monitors, and they were present on less than one-quarter of fishing trips (Observer Coverage).

Baseline: Before Catch Share Program

Otter trawl (Chart 1): During the baseline period, observed interactions were minimal or non-existent for most species. Observed interactions were extremely rare on small vessels using otter trawls. Although Chart 1 indicates a high rate of interaction for harbor porpoises and small vessels in 2004, only 1 harbor porpoise was involved; it appears as a large spike because of the low level of observed fishing effort. The highest average annual rate of interactions occurred between large vessels and whitesided dolphins, reaching a peak of 1.6 dolphins per 1,000 hours towing in 2004 before then declining. Compared to gillnets, otter trawls had a higher rate of interaction with whitesided dolphins. 

Gillnet (Chart 2): Observed interactions with gillnets were also generally low. Harbor porpoises had the highest rate of interaction with gillnet gear, particularly for small vessels using gillnets. During the baseline period, small gillnet vessels averaged approximately 0.029 interactions for every 1,000 net hours. Large gillnet vessels, by comparison, had one-tenth as many interactions on average. This difference likely arose from where and how the gear was used by large vessels compared to small vessels. Interactions with other species, such as common dolphin or humpback whale, were far less commonly observed. While there is some evidence of an increasing trend in interaction rates for small gillnet vessels during the baseline period, there was high variability among the years. The highest rate of interactions with harbor porpoises occurred in 2008. Only whitesided dolphin averaged more than one observed interaction per year, and the rates of interaction per unit effort were very low (0.001 for small vessels, 0 for large vessels).  

During Catch Share Program

Otter trawl (Chart 1): Otter trawl interactions with whitesided dolphins, the most commonly encountered species, were low in the first five years of the catch share program. One notable exception was in 2010, when on average 2.2 whitesided dolphins were encountered for every 1,000 hours of large-vessel otter trawling.

There was no apparent change in the frequency of whitesided dolphin interactions from the baseline to catch share period. During the catch share period, large vessels averaged 0.65 interactions per 1,000 hours of gear deployed, compared with a baseline period average of 0.73 interactions per 1,000 hours. However, there was some indication of increased rates of interactions with less commonly encountered species. Interaction rates of common dolphins and pilot whales for large vessels were higher during catch share program years than during the baseline period; however, the increase began just prior to implementation (2008-2009).  

We examined the spatial location of interactions with these species to determine whether the observed increase could be attributed to more observed trips occurring in areas where common dolphins and pilot whales were more vulnerable to fishing. Common dolphins were most commonly encountered during autumn and winter winter on Georges Bank (statistical areas 522, 525, and 562). Similarly, interactions with pilot whales were highest on Georges Bank and spread evenly across seasons. But there was no increase in observed fishing effort in these areas or seasons (Chart 3). Changes in locations and seasonal timing of fishing was likely not the only factor governing interactions with these species.

Therefore, for both common dolphins and pilot whales, there is no clear indication that broad-scale shifts in location of fishing activities of large vessels was responsible for the increase in common dolphin and pilot whale interactions. There is a suggestion that spatial distributional shifts may have contributed, but this would require more detailed analysis of spatial patterns of fleet behavior.

Gillnets (Chart 2): There was no apparent change in interaction rates between gillnets and protected species following implementation of the catch share program. Harbor porpoises were the most commonly encountered species. Average annual rates of encounters during the catch share years were nearly identical to the baseline period. However, the slightly increasing trend in interaction rates that was witnessed during the baseline period was reversed in the catch share program. 

Data Gaps and Limitations

We used data from the Northeast Fishery Observer program and At-Sea Monitoring Program to ask whether the rates of interactions (here defined as any observed interaction except for severely decomposed dead animal take) had declined following catch shares. Because vulnerability to fishing varies depending on the type of gear, and the way that gear was deployed, we examined each gear type and vessel size category separately. There was insufficient data on bottom longlining to examine that gear type, so only gillnets and otter trawls were examined. 

Information Sources

Data were kindly provided by the Northeast Fisheries Science Center.

Updated: August 2018

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