Underestimation of primary productivity on continental shelves: evidence from maximum size of extant surfclam (Spisula solidissima) populations

Abstract

Atlantic surfclams (Spisula solidissima), among the largest extant non‐symbiotic clam species in the world, live in dense aggregations along the Middle Atlantic Bight (MAB) continental shelf. The food resources that support these populations are poorly understood. An individual‐based model that simulates the growth of post‐settlement surfclams was used to investigate the quantity of food needed to maintain existing surfclam populations along the MAB continental shelf. Food inputs to the model were based on measured near‐bottom water‐column chlorophyll concentrations. Simulations showed that these water‐column food sources supported only 65% of the observed body mass of a standard large surfclam (160‐mm shell length). Additional simulations using benthic food sources to supplement water‐column food sources by 20% generated surfclams that grew to observed size and biomass and exhibited spawn timing consistent with the known surfclam spawning season. The simulation results suggest that measured water‐column chlorophyll concentrations may underestimate the food available to the continental shelf benthos. Large continental shelf bivalves are an essential resource for fisheries and higher trophic level consumers. Understanding available and utilized food resources is important for predicting long‐term impacts of climate change on benthic secondary production and fishery yield on the continental shelf.

Management strategy evaluation for the Atlantic surfclam (Spisula solidissima) using a spatially explicit, vessel-based fisheries model

“…Munroe et al (2016) found that the maximum size had, in fact, declined for much of the stock since 1980. Simulation modeling of Atlantic surfclam population dynamics shows that this outcome can be derived solely from rising bottom water temperatures (Munroe et al, 2013, although a change in food supply would result in the same outcome. Along the Mid-Atlantic coast, the Atlantic surfclam has supported a fishery since the 1960s that reached total revenues of $29 million in 2011 (Weinberg, 1999; Weinberg et al 5 ; NEFSC 2 ).…”

“…An additional survey conducted in 2004 (Weinberg et al 5 ) confirmed the northward and offshore shift in the Atlantic surfclam stock. One result of these mortality events was the redistribution of the stock north: namely an increasing abundance off the coast of Long Island, New York; the expansion of the population on Georges Bank; and the movement of the seaward boundary of the southern portion of the stock offshore in response to increased bottom water temperatures (Weinberg, 2005;Munroe et al, 2013; NEFSC 2 ). Simulations by Narváez et al (2015) based on stock assessment data from the Northeast Fisheries Science Center and bottom temperature time series obtained through implementation of the Regional Ocean Modeling System for the northwestern Atlantic indicated that episodic warm years caused elevated mortality events in older and larger clams and that these events have occurred with increasing frequency over the last several decades of the 20 th century.…”

“…Increased bottom water temperatures above approximately 20°C negatively affect Atlantic surfclam nutrition by reducing ingestion rate and leading to a reduction in growth rate, condition, and maximum size (Marzec et al, 2010;Munroe et al, 2013;Munroe et al, 2016). Munroe et al (2016) found that the maximum size had, in fact, declined for much of the stock since 1980.…”

“…The life span of Atlantic surfclam is approximately 30 years and has a maximum-recorded shell length (SL) of 226 mm (Fay et al 3 ; Cargnelli et al, 1999;Munroe et al, 2016). The range of Atlantic surfclam has been shifting north and offshore since approximately 1970, driven primarily by warming bottom water temperatures (Cargnelli et al, 1999;Weinberg, 2005;Munroe et al, 2013;Hofmann et al, in press). Early evidence of this trend is the disappearance of Atlantic surfclams in Virginia and Maryland state waters between the 1970s and the 1990s (Loesch and Ropes, 1977;Powell 4 ;Hofmann et al, in press) and the shift of the southern fishery from the Delmarva Peninsula to ports north .…”

“…Growth rates within the first 3 to 5 years 4 Powell, E. N. 2003 have been reported to be similar across much of the range of the Atlantic surfclam before the 1999 mortality event (Cargnelli et al, 1999). Increased bottom water temperatures above approximately 20°C negatively affect Atlantic surfclam nutrition by reducing ingestion rate and leading to a reduction in growth rate, condition, and maximum size (Marzec et al, 2010;Munroe et al, 2013;Munroe et al, 2016). Munroe et al (2016) found that the maximum size had, in fact, declined for much of the stock since 1980.…”