Estimation of Distributions of Lateral Fish Lengths from DIDSON Images

Dual-frequency identification sonar (DIDSON) has been used to estimate a large portion of daily salmon influx to the lower Fraser River near Mission, British Columbia.  Aside from migrating salmon, the acquired DIDSON data also contain images of resident fish species. Inclusion of resident fish in the counting will inflate the estimation of migratory salmon abundance.  At this time species differentiation using hydroacoustic data is not possible, but with the fish images collected by DIDSON we can group fish based on their lateral length distributions.  This allows us to estimate the relative abundance of the different sized fish in the river, so that we can exclude fish targets that are outside the typical adult salmon size distributions from the counting of salmon targets.     Length measurements, using the ‘Mark Fish’ application in the DIDSON software, are taken for fish images detected within a certain timeframe with good clarity.  The measured fish lengths are imported to a mixture model in the statistical program “R” to estimate proportions of salmon sized fish and small or large “resident” fish.  The salmon proportion is applied to the daily DIDSON fish count to obtain the daily salmon flux monitored by the DIDSON.  We have compared fish flux estimates between our split-beam and DIDSON sonar systems and found the estimates coming from both sonar systems to be very similar when a species proportion is applied to the DIDSON fish counts.