Research Document 2018/047

Relative strength of four cohorts (2012-15) of Atlantic Cod, from nearshore surveys of demersal age 0 & 1 juveniles in Newman Sound, Bonavista Bay

By Gregory, R.S., Dalley, K.L., Newton, B.H., Sargent, P.S, and Morris, C.J.

Abstract

We surveyed demersal age 0 and 1-year old Atlantic Cod (Gadus morhua) in the nearshore (<10 m deep) of Newman Sound, Bonavista Bay Newfoundland, during the 20 year period, 1995-2015. We used a seine net to conduct a qualitative assessment of the strength of these cohorts. Our assessment was based on comparisons with abundance of Atlantic Cod sampled at 6-12 sites, every two weeks from July until November, each year. Analysis of annual length frequency and abundance data indicated that age 0 Atlantic Cod settled into nearshore habitats in several distinct pulses, the first pulse arriving in early July in 2013 and 2015, but late in 2014 (i.e., September). Second and subsequent pulses followed the first by as much as a month and a half later, in each year. Strong pulse structure throughout the sampling period typically has resulted in the production of a strong cohort during previous survey years. The age 0 abundance in Newman Sound in 2013 and 2015 suggests that these two cohorts will also be moderate to strong, relative to other cohorts in the 20-year long time series. The 2013 cohort was the strongest in the 20-year time series in terms of mean abundance per set, timing of settlement, and strength of its recruitment pulse structure. Taking into account that two cohorts were sampled each year (as age 0 and age 1 individuals), this pre-recruit survey has shown that stronger than average cohorts were produced during each of the sampled cohorts of the four year period (2012-15), compared with other cohorts during the 20 years of monitoring. At age 1, the 2014 cohort was far weaker than we had predicted based on their age 0 abundance alone. We suggest that a combination of smaller sizes of individuals in this cohort at the end of the growing season in November, due to late initial settlement (September 2014), and protracted low temperature conditions overwinter may have contributed to lower overwinter survival to age 1, for the 2014 cohort. We anticipate that 2014 will be numerically weak.

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