Changes to Survey Indices and Implications for Assessment of Spiny Dogfish (Squalus acanthias) in the Northwest Atlantic

Abstract

Spiny Dogfish in NAFO areas 2–6 are considered to be one stock, with the greatest concentration of the population in US territorial waters. The main index of abundance used for assessment by the US and Canada derives from the National Marine Fisheries Service (NMFS) Spring trawl survey, where a new vessel with new monitoring protocols has been used in recent years. This document provides information on the data that will be used to assess Spiny Dogfish in Part II of a new Assessment Framework. It includes summaries of commercial catch and survey abundance indices relative to the dogfish stock definition. Also, it provides a comprehensive evaluation of factors that may influence dogfish catchability and discusses their effects to the NMFS Spring survey and their implications for population assessment. Differences in catchability owing to survey vessel, sampling strata, dogfish life stage, sex, day/night patterns, and combinations thereof were explored. A calibration approach specific to life stage is proposed to relate catches from the new survey vessel to the older one, and this results in a more biologically realistic trend in the abundance index for recent years. This calibrated index of stratified abundance at length will be compared using a split uncalibrated series in the stage-based population dynamics model in Part II of the Framework. Other changes to the structure of the assessment model suggested by the data were the need to incorporate sex-specific sampling error for the survey catches and to make process error proportional to the realized level of sampling in influential strata for Spiny Dogfish along the outer slope.