Bruce James McAdam

Evidence of Segregated Spawning in a Single Marine Fish Stock: Sympatric Divergence of Ecotypes in Icelandic Cod?

There is increasing recognition of intraspecific diversity and population structure within marine fish species, yet there is little direct evidence of the isolating mechanisms that maintain it or documentation of its ecological extent. We analyzed depth and temperature histories collected by electronic data storage tags retrieved from 104 Atlantic cod at liberty ≥1 year to evaluate a possible isolating mechanisms maintaining population structure within the Icelandic cod stock. This stock consists of two distinct behavioral types, resident coastal cod and migratory frontal cod, each occurring within two geographically distinct populations. Despite being captured together on the same spawning grounds, we show the behavioral types seem reproductively isolated by fine-scale differences in spawning habitat selection, primarily depth. Additionally, the different groups occupied distinct seasonal thermal and bathymetric niches that generally demonstrated low levels of overlap throughout the year. Our results indicate that isolating mechanisms, such as differential habitat selection during spawning, might contribute to maintaining diversity and fine-scale population structure in broadcast-spawning marine fishes.

Characterization of Atlantic Cod Spawning Habitat and Behavior in Icelandic Coastal Waters

The physical habitat used during spawning may potentially be an important factor affecting reproductive output of broadcast spawning marine fishes, particularly for species with complex, substrate-oriented mating systems and behaviors, such as Atlantic cod Gadus morhua. We characterized the habitat use and behavior of spawning Atlantic cod at two locations off the coast of southwestern Iceland during a 2-d research cruise (15–16 April 2009). We simultaneously operated two different active hydroacoustic gear types, a split beam echosounder and a dual frequency imaging sonar (DIDSON), as well as a remotely operated underwater vehicle (ROV). A total of five fish species were identified through ROV surveys: including cusk Brosme brosme, Atlantic cod, haddock Melanogrammus aeglefinus, lemon sole Microstomus kitt, and Atlantic redfish Sebastes spp. Of the three habitats identified in the acoustic surveys, the transitional habitat between boulder/lava field and sand habitats was characterized by greater fish density and acoustic target strength compared to that of sand or boulder/lava field habitats independently. Atlantic cod were observed behaving in a manner consistent with published descriptions of spawning. Individuals were observed ascending 1–5 m into the water column from the bottom at an average vertical swimming speed of 0.20–0.25 m s−1 and maintained an average spacing of 1.0–1.4 m between individuals. Our results suggest that cod do not choose spawning locations indiscriminately despite the fact that it is a broadcast spawning fish with planktonic eggs that are released well above the seafloor.

Identification of stock components using morphological markers.

This study investigated the development of a quantitative method for distinguishing stock components of Icelandic cod Gadus morhua based on visual examination of morphology. The stock is known to be structured into genetically distinct geographic components (north and south of Iceland) and behavioural types that spawn sympatrically. Differences in morphology were tested between locations, genotypes (a proxy for behaviour) and sexes. Results show morphological markers on the head, fins and body of G. morhua that are correlated with the sex, genotype of the fish at the pantophysin (pan-I) locus and the location at which the fish were caught. Females were found to have relatively deep bodies, and the pan-I(BB) genotype (associated with deep-water feeding behaviour) have greater gaps between their fins. Overall, morphology is more useful for distinguishing sympatric genotypes but less powerful at identifying genetically distinct geographic sub-populations, perhaps because counter-gradient evolution reduces phenotypic differences even with an underlying genetic cause.

Evidence from data storage tags for the presence of lunar and semi-lunar behavioral cycles in spawning Atlantic cod

Understanding the environmental processes determining the timing and success of reproduction is of critical importance to developing effective management strategies of marine fishes. Unfortunately it has proven difficult to comprehensively study the reproductive behavior of broadcast-spawning fishes. The use of electronic data storage tags (DSTs) has the potential to provide insights into the behavior of fishes. These tags allow for data collection over relatively large spatial and temporal scales that can be correlated to predicted environmental conditions and ultimately be used to refine predictions of year class strength. In this paper we present data retrieved from DSTs demonstrating that events putatively identified as Atlantic cod spawning behavior is tied to a lunar cycle with a pronounced semi-lunar cycle within it. Peak activity occurs around the full and new moon with no evidence of relationship with day/night cycles.

To glue or not to glue? Reassembling broken otoliths for population discrimination

Twenty-five Atlantic cod Gadus morhua otoliths were examined using eight shape measurements along with Fourier analysis of their outlines to test whether discrimination using otolith shape is affected by gluing broken otoliths. Small differences in seven of the eight shape measurements were found between unbroken otoliths and the same otoliths after breaking and subsequently gluing together; however, none of the Fourier descriptors differed. Cluster analyses indicated that resultant morphological differences will have no impact when applying discriminant analysis.