The Pacific Salmon Treaty recognizes the need to account for incidental fishing mortality and has instructed the Pacific Salmon Commission to "take into account such estimates of total chinook mortality in implementing the chinook rebuilding program" (paragraph 1(e) of the Chinook Annex). In 1987, the Chinook Technical Committee (CTC) reviewed agency reports and published literature on associated fishing mortality, and evaluated sources of associated mortality coastwide in marine and freshwater fisheries (CTC 1988). Since that report, substantial new information has become available on incidental mortality rates. This report reviews the information and recommends rates to be used for PSC fisheries in CTC analyses, and further recommends that fishery specific rates be applied as new information becomes available.

For hook-and-line fisheries, the CTC, recognized a range of hook-and-release mortality rates from 20 to 30% but applied a single mortality rate of 30% in both commercial troll and recreational fisheries. For net fisheries, the CTC recognized a range in mortality rates from 70 to 90%, but applied a single mortality rate of 90%. For the current report, the CTC considered commercial troll and recreational fisheries separately, and purse seine and gill net fisheries separately, because recent studies indicate that mortality rates can be substantially different between these gear types.

Commercial Troll. The CTC has been using a single hook and release mortality rate in the chinook model of 30% for commercial troll fisheries. For these fisheries, the CTC decided the available information supports using mortality rates that are specific for hook type (barbed, barbless) and fish size category (legal, sublegal). Total hook-and-release mortality rates recommended for these categories are:

The CTC also recommends calculating an additional mortality increment in CTC analyses for drop-off mortality. Drop-off mortality is composed of two components: (1) escaped encounters, fish that are hooked but escape from the gear; and (2) predation mortality, fish that are removed or lethally maimed by predators. Drop-off mortality is fishery specific, varying with hook type, escaped encounter rate, and predator encounter rate. Based on the current information available to the CTC, estimates of total drop-off mortality, expressed as a percentage of chinook brought to the boat (including both those retained and those released), are:

SEAK Troll (barbed hooks): 0.8%

Oregon Troll (barbless hooks): 2.5%

Average (barbed hooks): 1.7%

Most of the variability in the rates developed by the CTC was due to differences in predator encounters. Because drop-off mortality is calculated as a percentage of both retained and released chinook salmon, the drop-off mortality rate is only additive to the hook-and-release mortality rate (which is a percentage of the fish released) for non-retention fisheries, when both legal and sublegal fish are released. In troll fisheries where legal-sized chinook are retained, total non-landed mortality must be calculated as two different components: hook-and-release mortality of sublegal fish, and drop-off mortality of both legal and sublegal fish.

Recreational. For recreational fisheries, the CTC found large differences in hooking mortality between fishing techniques. Recent research has documented high mortality rates associated with "California mooching" fishing method. At this time, the CTC agreed that rates for hook-and-release mortality for recreational fisheries of concern to the PSC should not incorporate data from the California mooching studies. However, if this fishing technique becomes more common in PSC fisheries, then the CTC estimates will need to be revised accordingly.

The CTC has been using a single hook and release mortality rate in the chinook model of 30% for recreational fisheries. The CTC considered adjustments to mortality rates due to hook type (barbed/barbless) and size category. Because the available data are not conclusive or consistent on the effect of hook type, the CTC decided not to consider barbed/barbless hook differences for estimating hook-and-release mortality in recreational fisheries. The CTC decided that the available data supported different mortality rates for chinook salmon < 33 cm, but no size-related differences among larger. Because chinook < 33 cm are not common in PSC fisheries, the rate for the larger size class will be used, unless size distribution data from a particular recreational fishery indicates that the rate should be weighted for the proportion of small fish present. The CTC recommends for CTC model analyses the following total hook-and-release mortality rates for recreational fishing specific for the two size categories:

>33 cm : 12.3%

<33 cm : 32.2%

The CTC also recommends calculating an additional mortality increment for drop-off mortality in CTC analyses of recreational fisheries. This rate, based on the total chinook boated (landed catch and released catch), is fishery specific, varying with escaped encounter rate and predator encounter rate. Based on the current information available to the CTC, the following fishery specific rates are recommended:

SEAK: 3.6%

Puget Sound: 14.5%

Oregon: 2.7%

For fisheries where specific data are not available, the CTC will use the average of these rates, 6.9%, to estimate escaped encounters and predator losses.

Purse Seine. The CTC has been using a total non-retention mortality rate of 90% in the chinook model for purse seines. Recent studies indicate that chinook salmon mortality rates are potentially much lower for fish released from purse seines. The CTC recommends using a rate of 72% for total chinook non-retention mortality for all size classes of fish combined in CTC analyses. This rate is based on an average immediate mortality of 49% and a delayed mortality of 23%.

When size-distribution information is available, the CTC recommends using the following estimates of immediate and total non-retention mortality:

Immediate Total

small chinook: 62.8% 85.8%

medium chinook: 50.5% 73.5%

large chinook: 28.0% 51.0%

In terminal fisheries where chinook salmon are released and a specific portion are close to spawning (< 60 d), the delayed mortality should be weighted to account for the lower rate of 1.1% observed for such fish.

The CTC will use these rates as generalized estimates of purse seine non-retention mortality. However, immediate mortality is fishery- and time-specific because of such factors as the type of fishery, frequency of catch sizes (related to handling time and suffocation of the chinook), incidence of chinook in sets by time and area of the fishery, handling of the chinook during boarding, and size distribution of the chinook caught. When fishery specific information becomes available for particular model fisheries, the CTC will apply the more specific estimates.

Gill Net. Prior to this review, the CTC assumed 90% release mortality for gill nets. This mortality rate has not been applied in the chinook model, because chinook non-retention (CNR) restrictions in gill net fisheries have not been explicitly modelled. CNR gill net fisheries are unlikely to be implemented due to high expected mortality rate of the fish released. However, CNR periods are likely in gill net fisheries where a limit on total chinook landings is imposed.

Review of the available literature indicated that gill net release mortality rates can be highly variable and may be substantially lower than 90% for salmon in their final year of life and close to maturity. Chinook salmon released from gill-nets in chinook model fisheries, however, are likely to be small, immature fish. Until data are available for this age and size range, the CTC will use the 90% mortality rate previously assumed.

The drop-out mortality rates currently used in the chinook model were derived from the available literature. This a poorly investigated subject; the rates must be viewed as very uncertain. Rates are expected to vary from fishery to fishery due to variables such as mesh size, prevailing weather and sea conditions, and predator abundance. These rates will continue to be used, however, pending review of updated incidental mortality reports from the agencies. Gill net fisheries occur primarily in SEAK, Fraser River, Puget Sound, the Washington Coast, and Columbia River. Until better information is available, the CTC will use the following drop-out mortality rates for these fisheries:

SEAK: 2%

Fraser River: 8%

Puget Sound: 8% (includes some purse seine fisheries)

Washington Coast: 2%

Columbia River: 3%