Population dynamics ofArctica islandicaat Georges Bank (USA): an analysis of sex-based demographics

Abstract

The ocean quahog, Arctica islandica, is a commercially important bivalve in the eastern USA but very little is known about the recruitment frequency and rebuilding capacity of this species. As the longest-living bivalve on Earth, A. islandica can achieve lifespans in excess of 200 y; however, age determinations are difficult to estimate and age variability at size is extreme. Objectives for this study included the creation of an extremely large age-composition dataset to constrain age at length variability, development of reliable age-length keys (ALK), and descriptions of sex-based population dynamics for the quasi-virgin A. islandica population at Georges Bank (GB) within the greater US Mid-Atlantic stock. Sexually dimorphic characteristics are clearly present, as females are larger than males within age classes and males tend to dominate the oldest age classes. A male represented the maximum age of 261 years and is older than the maximum age previously documented for this region. Sex-specific ALKs were robust and reliable but not interchangeable. This population had higher estimated natural mortality rates than presumed for other regions in the Mid-Atlantic, and females have the highest mortality rate. However, recruitment expansion was also occurring which would affect the age-frequency data used to derive mortality estimates and result in higher mortality. Age frequencies at GB suggest effective recruitment to the population each year since 1867 CE. Reduced recruitment periods are documented and likely attributed to fluctuating environmental conditions. Sex-based demographics are clearly divergent in regard to growth rate, maximum size, longevity and mortality rates.

Historical biogeographic range shifts and the influence of climate change on ocean quahogs (Arctica islandica) on the Mid-Atlantic Bight

“…Marine reservoir age residuals obtained by comparing the global marine reservoir age of 400 years from Marine13 (Reimer et al, 2013) with the birth dates determined by visual aging from a sample of live-caught radiocarbon-dated Arctica islandica obtained from Georges Bank (40.72767°N 67.79850°W, 72.5 m; Hemeon et al, 2021), Long Island (40.09658°N 73.01057 W, 47.5 m; Pace et al, 2017b), and northern New Jersey (39.33°N 73.12°W, 62.5 m).…”

“…In addition, the majority of earlier estimations for the North Atlantic come from the eastern Atlantic, greater depths, or higher latitudes, with mixes of water sources divergent from the composition of the Cold Pool (Ascough et al, 2007; Eiríksson et al, 2004; Heaton et al, 2020; Sherwood et al, 2008; Tisnérat-Laborde et al, 2010). Given the absence of a regional MRA in the MAB offshore of the Delmarva Peninsula, a series of live A. islandica were obtained from sites off New Jersey, Long Island, and Georges Bank as described by Hemeon et al (2021) and Sower et al (2022). Shells from live animals were processed and radiocarbon dated using the same method followed for dead shells.…”

“…The NEFSC survey only encompasses the last 40 years of the present warming period beginning in the 1800s (Pace et al, 2017a(Pace et al, , 2017b. Regional population age frequencies suggest A. islandica existed in relatively low abundance over its present biogeographic range during the late LIA, followed by a rapid population expansion in the late 1800s throughout the MAB (Hemeon et al, 2021;Pace et al, 2017b) with high population abundance continuing through today. During the LIA and through the 1800s, colder water presumably extended closer inshore, supporting the inshore range of ocean quahogs documented by the distribution of dead shells.…”

Defining Patterns in Ocean Quahog (Arctica islandica) Sexual Dimorphism along the Mid-Atlantic Bight

“…In comparison to gastropods, sexual dimorphism in bivalves is rare (Sastry 1979) with cases of protandry and sequential hermaphroditism being most common [e.g., some oysters (Orton 1927, Coe 1934, Dinamani 1974; pearl oysters (Chávez-Villalba et al 2011); and arc shells (Peharda et al 2006)]. Cases of dwarf males are rarer still (teredinids, Turner & Yakovlev 1983); however, evidence of size-based sexual dimorphism has been found in A. islandica (Ropes et al 1984a, Fritz 1991, Steingrímsson & Thórarinsdóttir 1995, Thórarinsdóttir & Steingrímsson 2000, Hemeon et al 2021.…”

“…As Arctica islandica age, males and females consistently display distinct differences in overall size: females tend to dominate size classes larger than those of males (Ropes et al 1984a, Hemeon et al 2021. Based on *Corresponding Author.…”