Patterns and implications of skip-molting for the Eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

A novel morphometry-based method for assessing maturity in male Tanner crab, Chionoecetes bairdi

“…It is important to note that the use of this cutoff in the DID method was more a formality than an ironclad requirement to maintain analytical validity. This issue also seems apparent in other applications of the mixture of regressions approach, and is thus not an artifact of our specific implementation (Murphy 2019). While the DID cutline does classify more large CW, small-clawed crabs as being mature than the mixture of regressions cutline, this error is much easier to resolve during the analysis process than when small CW small-clawed crabs as classed as mature, not least because these cases will be much more apparent.…”

“…Consequently, these procedures may be problematic to apply in cases where the investigator lacks samples with known maturity statuses from which initial conditions for the given classifier may be derived (Warren 1994;Rugolo et al 2005). In response, mixture of regressions procedures have been developed and applied to crustacean data for maturity classification on an exploratory basis (Warren 1994;Turner 2000;Rugolo et al 2005;Murphy 2019). This approach, however, requires carefully selected initial values for the model parameters, and the investigator may be required to follow an iterative analysis process involving many runs, capped by a subjective decision as to the final "best fit".…”

Sub-Arctic no more: Short- and long-term global-scale prospects for snow crab (Chionoecetes opilio) under global warming

“…This invoked an assumption that most crab in the exploitable biomass in any given stock were 10-13 years of age. This assumption was based on including additional years above the mimimum 9 years identified by [41,42] to reach 95mm CW to account for skip-molting in these populations [43][44][45] and is supported by recent work on age determination in male snow crab in the NL stock which showed them to average 10-12 years of age when first recruiting to exploitable size [46]. A proportion of the exploitable biomass in each Stock Region would normally consist of residual crab, present in the population for a year or more, but different fisheries exploitation rates across Stock Regions would affect temporal signals in lag periods for relationships in this long-term outcome stage.…”

“…We therefore infer that a recent and spatially broad-based ocean climate event likely promoted the collapse. This inference is based on a key observation that all stock components including both sexes and all instar and maturity stages were adversely affected at once, with habitat partitioning known to exist among these different population components over large spatial scales in the Eastern Bering Sea [19,44]. In specific reference to the recent unexplained trajectory of the sGSL stock, beyond the possibility that recent upward biases in stock size measurements could be occurring [9], we suggest that release from cod predation [66] could be a factor progressively enabling continued high productivity in the stock, with extremely low levels of cod biomass for over a decade and a possible extinction of cod from that ecosystem within decades [39].…”

Older but not wider. Impacts of molt frequency variation on age and growth dynamics of Newfoundland & Labrador snow crab (Chionoecetes opilio) toward corroborating gastric mill 2 age estimations

“…Following a frequent but progressively slowing molt rate in earliest post-settlement ontogeny, a near-annual molting schedule often becomes established over the instar VI (Sainte-Marie et al, 1995;Comeau et al, 1998) to instar VIII (Yamamoto et al, 2015b) range, depending on environmental conditions. Thereafter, skip-molting of annual molts by males becomes commonplace in some populations, particularly in moderately-large adolescent males (Taylor et al, 1994;Hébert et al, 2002;Dawe et al, 2012;Yamamoto et al, 2018b;Murphy, 2019. In populations where this occurs, it can lead to average molt frequency rates beyond about instars VII-VIII becoming less than annual.…”