Salmon in the Strait

From  ”State of physical, biological, and selected fishery resources of Pacific Canadian marine ecosystems in 2012

Chrys Neville, Rusty Sweeting, and Dick Beamish have carried out surveys for juvenile salmon in the Strait of Georgia every summer (June/July) and fall (September) since 1998. Data from these midwater trawl surveys are used as proxies to forecast marine survival trends for that species and brood year.

Juvenile salmon generally enter the Strait of Georgia from April to June, and many stay there until the fall. These researchers use both the catch rates from their trawl surveys, along with information on the oceanographic conditions in the Strait of Georgia to estimate the relative strength of returning adults (return year dependent on species) for juvenile salmon that entered the ocean.

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Figure 1. Average surface water temperature (SST) in May-July 1960-2012 in the Strait of Georgia. Measurements are averaged from five locations throughout the strait, including Cape Mudge, Chrome Island, Departure Bay, Entrance Islands, and Sisters Island. Data available from http://www.pac.dfo-mpo.gc.ca/science/oceans/data-donnees/lighthouses-phares/index-eng.htm. The red bars are regime averages.<

 

The surface water temperature (SST) of the Strait of Georgia has been generally in a cooling trend in the past few years (Figure 1), while surface salinity has also been slightly lower over the past few years due to high discharge from the Fraser River. The researchers not that cooler SST and lack of extreme flow conditions is generally associated with high productivity conditions in the Strait of Georgia for juvenile salmon.

 

 

 

 

 

 

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Figure 2. (A) CPUE and (B) anomaly of average length for juvenile Sockeye Salmon caught in the June/July trawl surveys in the Strait of Georgia 1998-2012. The juveniles in 2012 are from the record adult return to the Fraser River in 2010. Dashed line in (A) is average CPUE for 1998-2012.

Many of the juvenile Sockeye Salmon that entered the Strait of Georgia in 2012 were progeny of the record return of Sockeye Salmon to the Fraser River in 2010. It was, therefore, not surprising that the CPUE of juvenile Sockeye Salmon in the early summer survey was the highest in the time series and almost double the long term average (Fig. 2A). While the length of these fish was the smallest observed in the time series (Fig. 2B), the size of sockeye juveniles in 2008, which resulted in the large 2010 return, was also small. Therefore, if there is no additional impact of the small size and if marine conditions are favourable along their subsequent migration route, the researchers expect a good return of sockeye in 2014. 

 

 

 

 

 

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Figure 3. CPUE of Coho Salmon in (A) June-July and (B) September 1998-2012 in the Strait of Georgia. Dashed lines are average CPUE for 1998-2012

Coho salmon generally spend one winter in the ocean, therefore, most juveniles that entered the ocean in 2012 will return in 2013. Juvenile Coho Salmon generally remain in the Strait of Georgia until fall (Chittenden et al. 2009) and this region is important in determining brood year strength (Beamish et al. 2010). The CPUE of Coho Salmon in July 2012 was similar to 2011 and below the time series average (Fig. 3A). However, the CPUE of Coho Salmon in the September 2012 survey was the highest in the time series (Fig. 3B) and the estimated abundance of Coho Salmon in the Strait of Georgia in September 2012 was 4.0 million juveniles. The researchers also expect a good return of Coho Salmon in 2013.

 

 

 

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Figure 4. CPUE of Chum Salmon in the June-July trawl surveys in the Strait of Georgia 1998-2012. Dashed line is average CPUE for 1998-2012

Chum salmon enter the ocean in the year they emerge from the gravel and typically spend three winters in the ocean. The CPUE of Chum Salmon that entered the ocean in 2012 and were caught in the early summer survey was very low and similar to 2011 (Fig. 4). These two years were two of the four lowest observed in their time series, and the juveniles were small both years. Based on these results, the researchers expect poor returns of Chum Salmon in both 2014 and 2015.

 

 

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Figure 5. (A) CPUE and (B) anomaly of average length of juvenile Chinook Salmon caught in the June/July trawl surveys in the Strait of Georgia 1998-2012. Dashed line in (A) is average CPUE for 1998-2012.

Chinook Salmon early life history is complex with variation in life history type (ocean and stream-type), age at ocean entry and timing of ocean entry. Results of DNA analysis of Chinook Salmon described in Beamish et al. (2011) indicated that catches in the June/July surveys are a mixture of ocean and stream type Chinook Salmon. The CPUE of Chinook Salmon that entered the ocean in the spring/early summer of 2012 and were caught in the early summer survey continued a declining trend from 2006 and was below the long term average (Fig. 5A). The average size of these fish was the largest that had been observed in the time series (Fig. 5B) but there may be different reasons for this.  The researchers expect continued poor marine survival for Chinook Salmon and poor returns in 2014 and 2015.

 

 

 

 

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Figure 5. CPUE of Pink Salmon caught in the June/July trawl surveys in the Strait of Georgia 1998-2012. Dashed line is average CPUE for 1998-2012 excluding 2010.

The CPUE of Pink salmon in the early summer survey in 2012 was similar to observations in 2000 to 2008 (Fig. 5). It was not near the record high catches observed in 2010 although moderately strong related to the other even numbered years. The researchers expect to see good returns of Pink Salmon to the Fraser River in 2013.