Green sturgeon (Acipenser medirostris) COSEWIC assessment and status report

• Previous page
• Table of contents
• Next page

Population sizes and trends

• Canadian assessment
• U.S. Assessment
• Asian population of Sakhalin sturgeon

A comprehensive population size and trends analysis has not been done for the green sturgeon population in Canada. The first Committee on the Status of Endangered Wildlife in Canada (COSEWIC) report in 1987 classified the species as rare due to a lack of information on the habitat requirements and biology of the species in addition to a paucity of information on its population status (Houston 1988). Unfortunately, this has not substantially improved. Information on the green sturgeon found in Canada is largely anecdotal prior to 1996; as Department of Fisheries and Oceans Canada (DFO) catch statistics only began differentiating green and white sturgeon with the implementation of 100% fisheries observer coverage for the domestic trawl fishery after this date. No spawning populations in Canada are known, and there is no evidence that any existed historically. The lack of studies on the green sturgeon in Canada is largely due to its rarity, its low commercial value and the relative ease of misidentification with the more common white sturgeon. However, the absence of evidence is not evidence of absence and a few green sturgeon have been caught in areas where spawning might occur. While there has been intensive sampling for white sturgeon, the same is not the case for the green surgeon, and no specific attempts have been made to look for adults, let alone young-of-the-year or juveniles. Work by Israel et al. (2002) suggests that there may be a discrete spawning population yet to be discovered; it is considered unlikely, but not impossible that this population is spawning in Canada despite the lack of evidence.

The vast majority of information regarding the habitat needs, biology and population status and trends of green sturgeon comes from the US. where all currently known spawning populations are located.

Canadian assessment

Green sturgeon are generally restricted to marine environments in Canada; however, there is anecdotal evidence that suggests that they may have historically utilized the lower portions of the Fraser, Skeena, and Nass rivers (McPhail and Carveth 1993). Due to their similarity to white sturgeon, there is the possibility that green sturgeon may have been substantially impacted during the collapse of the unregulated white sturgeon fishery in the late 1800s when the white sturgeon fishery was declared “practically commercially extinct” by a fisheries inspector in 1902 (EPIC 2001). Because of their similarity and range overlap with white sturgeon, green sturgeon were likely caught in the fishery and discarded or sold without being reported in landing statistics. The possible impact on the population will likely never be known as historical catch statistics differentiating between sturgeon species do not exist. There is the possibility that they may have been somewhat protected due to their disagreeable taste and low market value. Large marine catches, summer estuarine concentrations, and a possible negative attitude towards releasing the fish alive due to the feeling that they may be displacing white sturgeon habitat makes it impossible to assess the relative impact that green sturgeon may have suffered during the unregulated white sturgeon fishery.

Green sturgeon have been encountered in freshwater in Canada through white sturgeon tagging studies. In 1986, a tagging study captured and tagged 500 white sturgeon, and two green sturgeon (which were not positively identified but simply appeared as “different” from white sturgeon) between approximately 50 to 90 km upstream from the Fraser River mouth (Houston (1988). A similar study, which overlapped a portion of the original study area spanning the years 1995 to 1999, tagged 414 white sturgeon between Mission (rKm 78) and Bristol Island (rKm 154) but failed to find any green sturgeon. An additional 1429 white sturgeon were captured during the study farther upstream from this area with no green sturgeon being found (Adams et al. 2002). The lower Fraser River has had more reports of green sturgeon; however, they continue to be rare. Nearly 13 000 white sturgeon have been tagged from the beginning of 2000 to June 2003, and 12 to 15 possible green sturgeon have been reported (Nelson pers. comm.). However, as green sturgeon in Canadian waters are generally marine residents, freshwater encounters were likely always rare with the majority of catches being restricted to marine commercial fishing vessels. The first documented catch of green sturgeon on the coast of British Columbia (BC) occurred in 1954 off Spring Island near Kyuquot Sound. Seventy-five green sturgeon were caught in one trawl haul at a depth of 78.6 m. The average length and weight of these fish was 119.4 cm (range 94 to 203 cm) and 12.7 kg, respectively (Anonymous 1954).

A survey was conducted in 1995 by Slack and Stace-Smith (1996) due to a concern over the absence of data about the population status and a concern that the population was on the decline. The survey included local commercial, longline, gillnet and sport fishers who were asked about any current or historic encounters with green sturgeon. The survey used notices posted at fishing docks at Stevenston and False Creek, calls at fishers’ meetings and a notice in The West Coast Fisherman magazine. The results of the survey indicated that in the early 1980s, some large trawlers off the west coast of Vancouver Island caught and released large numbers of green sturgeon; however, large catches were not observed from 1988 to 1995. Furthermore, since 1985, the Pacific Salmon Commission has only found one green sturgeon in all test fisheries that were conducted, and it was unconfirmed (J. Gable, Racial Identification Group, Pacific Salmon Commission; pers. comm. 2003). In the bottom trawl fishery, large single hauls of green sturgeon reaching 9000 kg were reported in 1960 and catches from 1737 to 4500 kg were being landed between 1989 and 1992. Moreover, green sturgeon were incidentally caught in salmon gillnets at the mouths of large rivers (Table 2). In addition to the observations reported in Table 3, deliveries to a fish processing company in Port Hardy reported some small catches (2 to 6 per boat) between January 1994 to June 1995, which were sold dockside or taken home by the crew.

Weights are catches from single large bottom trawls and individuals were caught from either gillnets or longline. Individuals are reported at an estimated weight of 30 kg (Houston 1988).

The survey conducted by Slack and Stace-Smith (1996), although anecdotal and incomplete, is the only substantial source of information regarding historic catch sizes of green sturgeon prior to 1996. The survey suggests that green sturgeon catches may have declined since the early 1960s. The survey data, however, needs to be viewed cautiously and is of limited value for stock assessment purposes. For instance, the data does not reflect the overall decline in total commercial fisheries which has occurred since the 1960s. This would result in lower effort and likelihood of sturgeon captures. Additionally, there is no method to ascertain the accuracy or precision of catch weight estimation or whether the survey information incorporates all green sturgeon catches, as fishers who had left the fishery or not responded to the survey would be omitted. Furthermore, temporal standardization of fishing effort is absent. Therefore, the catch statistics formulated through this survey of single abnormally large catches do not reflect total annual catches of green sturgeon.

A method to develop historical catch estimations of green sturgeon is possible by estimating species composition of undifferentiated sturgeon catches between those in marine and freshwater. As green and white sturgeons are generally marine and freshwater residents respectively in Canadian waters, by observing catch location, it may be possible to develop a historical time series of green sturgeon catches. Estimations would include a degree of error as white sturgeon have been caught in marine waters and green sturgeon in freshwater. Furthermore, the data is currently not readily available in electronic form and would be difficult to compile (J. Echols, Selective Fishing Co-coordinator, Department of Fisheries and Oceans, Vancouver, BC; pers. comm. 2003).

Quantitative data on green sturgeon catches in Canada is limited as DFO only started differentiating between green and white sturgeon in their catch statistics after 1996 with the initiation of 100% observer coverage for the domestic trawl fishery. Catches of green sturgeon are rare and single catches generally make up a substantial portion of the entire yearly catch as can be seen from the upper range of the biomass per trawl (Table 3). In each year, a single haul accounts for 25 to 52% of the total yearly catch.

From 1996 to 2002, the catches were composed of 171 green sturgeon (mean weight = 65.6 kg). A total of 151 138 sets were recorded for the fishery over this time indicting that green sturgeon were caught in only 0.07% of catches (Levings and Nelson 2002). The low incidences of green sturgeon over the span of the data set indicate that green sturgeon catches are rare. Catch data is not standardized for overall fishery effort or gear selectivity which may have changed over the time series, making the data presented here of limited value. Standardized data, however, would be of little use in determining population sizes and trends due to the short time series and relatively small and highly variable catch sizes. Therefore, making any reasonable quantitative assessment of the status and trends of green sturgeon found in Canadian waters is not practical at this time.

The reports of large single catches up to 9000 kg and many significantly over 1000 kg as reported by Slack and Stace-Smith (1996) have not been observed since 100% observer coverage for the domestic trawl fishery was instituted in 1996. This may be an indication that the green sturgeon population has suffered a decline over the past few decades; however, this claim cannot be made with certainty. 

U.S. Assessment

The National Marine Fisheries Service (NMFS) recently conducted a status review of green sturgeon (Adams et al. 2002) to determine if the species should be listed as an endangered or threatened species under the US Federal Endangered Species Act

(EPIC 2001). The status review determined that neither the northern, nor the southern green sturgeon discrete population segments (DPSs) warranted listing as threatened or endangered at this time (Adams et al. 2002); although it was acknowledged that the northern DPS could be in the foreseeable future (Adams et al. 2002). The best evidence available seems to indicate that the green sturgeon has undergone significant range contraction and in turn has likely suffered a substantial decline in abundance.

Asian population of Sakhalin sturgeon

The Sakhalin population of green sturgeon has been extirpated throughout Japan, Korea, and China and is reduced in range in Russia to the Tummin River where a hatchery maintains its population (EPIC 2001). It is listed as endangered on the International Union for Conservation of Nature (IUCN) red list (IUCN 2002). The taxonomic relationship between the North American and Asian form (the Sakhalin sturgeon) of green sturgeon is still in debate.