Science Response 2014/046

Effects of Geoduck biological sample handling and transport time on mean weight estimation

Context

The commercial fishery for the Pacific Geoduck Clam (Panopea generosa) began in British Columbia (BC) in 1976 and has since grown to be one of the highest valued fisheries in BC at CAD $46.6 million in 2012 (BC Agriculture 2012).

The Geoduck fishery in BC is managed at the level of the individual Geoduck bed. Geoduck biomass is estimated for each bed as the product of bed area, Geoduck density on the bed, and mean Geoduck weight for the bed. Harvest options are based on regional annual exploitation rates of 1.2 – 1.8 % applied to the estimated current biomass for each bed (Bureau et al. 2012). Since 2001, mean Geoduck weight has been estimated from fishery-dependent data derived from numbers and weights of landed Geoducks recorded in commercial harvest logbooks (Hand and Bureau 2012; Bureau et al. 2012). Fishery-independent dive surveys are conducted to estimate Geoduck density on the beds, which historically included the collection of biological samples for age determination. Geoduck biological samples thus provide an additional source of mean weight information for some surveyed beds.

A number of reasons make commercial harvest data preferable over biological sample data for Geoduck mean weight estimation (Bureau et al. 2012). Commercial harvest data offer better spatial representation within a bed (i.e., harvest from more locations) and a larger number of Geoducks within a bed than biological samples. The larger sample size in commercial catch is expected to produce more precise estimates of mean weight. Commercial harvest data also provide bed-specific mean weight estimates for a greater number of beds than do biological samples. Mean weight estimates from commercial harvest data are updated yearly while few biological samples are collected in a given year. Timing between commercial harvest, recruitment events and biological sampling dates can lead to differences in mean weight estimates. The increased temporal resolution of commercial harvest data is also better able to capture variations in recruitment events that can lead to higher (or lower) estimates of mean weight.

Calculations of Geoduck harvestable biomass and fishery harvest options must rely on estimates of mean weight that are representative of Geoducks counted during density dive surveys. Concerns over the representativeness of commercial catch data to estimate Geoduck mean weight were raised because juvenile Geoducks are sometimes too small to be seen by harvesters and because of concerns over size selectivity in the fishery (Hand and Bureau 2012). However, some juveniles that are too small to be seen by harvesters may also be too small for survey divers to see. Bureau et al. (2012) showed that mean weights estimated from biological samples were lower than mean weights derived from commercial catch data for the Haida Gwaii and Prince Rupert regions while no differences were observed for the Central Coast and South Coast. Therefore, based on observed differences between biological sample and commercial catch mean weights, correction factors of -10% and -8% were recommended to be applied to the fishery-dependent mean weights for Haida Gwaii and Prince Rupert, respectively, until further work was conducted. It was recognized that Geoduck biological samples are subject to more handling and longer transport times than the commercial catch, and therefore biological samples could be subject to greater water loss (i.e., weight loss) prior to being weighed. However, due to lack of data, analyses presented in Bureau et al. (2012) did not take into account the possible effects of increased handling and transport time on biological sample mean weight estimates.

Fisheries and Oceans Canada’s Pacific Region Fisheries Management Branch requested Pacific Region Science Branch to provide science information and advice to answer the following questions:

1. Does handling and transport time of Geoduck biological samples account for differences in mean weight estimates observed between biological samples and commercial catch data for the Haida Gwaii and Prince Rupert regions?
2. When handling and transport time of Geoduck biological samples are accounted for, are there still differences between mean weight estimates from biological samples and the commercial catch for the Haida Gwaii and Prince Rupert regions?
3. Are correction factors still required for mean weight estimates derived from commercial catch data from Haida Gwaii and Prince Rupert regions?
   

An experiment was conducted to determine if handling practices and transport time of Geoduck biological samples can affect mean weight estimates (Bureau and Curtis 2014).  Results indicated that increased handling and longer duration between collection and weighing at a landing dock result in greater weight loss in biological samples compared to commercial catch. Correction factors, based on empirical transit time data and results of the experiment, were applied to biological sample data from 1994 to 2010 for Haida Gwaii and Prince Rupert regions.  There were no statistically significant differences between corrected mean weights from biological samples and mean weights from commercial catch data for Geoducks from the Haida Gwaii and Prince Rupert regions. Therefore, correction factors are no longer recommended for fishery-dependent mean weight estimates for the Prince Rupert and Haida Gwaii Regions.  Mean weight for the purpose of Geoduck biomass calculation can be estimated directly from commercial logbook data.

This Science Response Report results from the Science Response Process of June 2013 on the effects of Geoduck biological sample handling and transport time on mean weight estimation.  This Science Response replaces previous science advice on this issue provided in Bureau et al. (2012).  At this time, there is no expectation of further advice on this issue.

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