What is a spawning aggregation?
Spawning Aggregations – Definitions, Scales, Types, and others
Basic Definition (from Domeier 2012):
A spawning aggregation is a repeated concentration of conspecific animals, gathered for the purpose of reproduction, that is predictable in time and space and results in a mass point source of offspring. The density/number of individuals participating in a spawning aggregation is at least four times that found outside the aggregation.
- Requires migration and increase in density/abundance
- Meant to exclude simple migrations of small groups (ie. Simple Migratory Spawning – migration of pairs or small groups of fishes from a non-spawning to a spawning area; Colin and Domeier 1997).
- Meant to exclude a pair spawn plus a streaking (parasitic) male
- Meant to distinguish a phenomenon where an entire subpopulation of individuals halt their normal routine, migrate, gather and spawn.
- The definition and criteria was intended to capture the conservation/fisheries component related to increased catchability leading to a target for fishing (due to large concentrations, predictable in time and space).
- Excludes sites for copulation (e.g. elasmobranchs) since they no offspring are released, but single sex gatherings for the purpose of releasing offspring (e.g. decapods) are considered spawning aggregations.
Types of Spawning Aggregations (from Nemeth 2009; Domeier 2012):
Resident spawning aggregations draw individuals to a site within or nearby their adult home range. They usually (1) occur at a specific time of day over numerous days, (2) last only a few hours or less, (3) occur daily over an often lengthy reproductive period of the year; and (4) can occur year round. A single day of spawning for an individual participating in a resident spawning aggregation represents a small fraction of that individual’s annual reproductive effort.
Transient spawning aggregations draw individuals to a site well outside their typical adult home range. Transient spawning aggregations often (1) occur during a very specific portion of one or two months of the year; (2) persist for a period of days or at most a few weeks and (3) do not occur year round. A single transient spawning aggregation may represent the total reproductive effort for participating individuals.
- Functional differences to justify separation of resident and transient beyond just scale: separation falls along phylogenetic lines and trophic levels (predators are transient, herbivores mainly resident)
- Acknowledgement existence of exceptions and the fact that many show intermediate patterns (partial migrations – e.g. leopard coraltrout).
- Schooling fishes are known to join conspecific schools at predictable sites and times to form both transient (jacks, mackerels, tunas, croakers) and resident (wrasses, parrotfishes). The movement of a single school of animals to a specific site so spawn falls under the definition of Simple Migratory Spawning.
Egg Types and Spawning Aggregations
- Most release pelagic eggs
- Some are demersal spawners (e.g. Balistids, pomacentrids, siganids)
- Can include those w/ or w/o parental care
- Copulating/live bearers/elasmobranchs – see definitions
Spawning Aggregations in non-reef habitats
Pelagic fishes and migratory coastal species at higher latitudes (e.g. temperate) appear to be more likely to exhibit less spatial precision than demersal species. Croakers, herrings/sardines, mullets and flyingfishes are examples of fishes that form predictable spawning aggregations on a larger spatial scale.
Factors associated with Vulnerability to fishing (from J. Robinson’s thesis)
- Life history parameters : longevity, age at maturity, asymptotic length show positive relationship with vulnerability. Growth parameter (K) and natural mortality (M) show a negative relationship with vulnerability.
- Spawning Season (1-4): shorter spawning seasons confer higher vulnerability to aggregation fishing.
- Aggregation type (1-4): resident (1-2) and transient (3-4) types. Resident types distinguished by migration range, spatial concentration, distinctiveness of spawning sites. Transient types distinguished by spatial concentration and distinctiveness of spawning sites.
- Density/Abundance Change (1-6): Order of magnitude comparisons between peak spawning densities and normal, non-spawning densities. Scale distinguishes between species that are solitary, grouping or schooling for non-reproductive functions. Larger density changes confer higher vulnerability to aggregation fishing.
- Aggregation duration (1-4): scales the duration of spawning aggreagtaions ranging from less than one day to more than two weeks, with longer durations conferring higher vulnerability to aggregation fishing.
- Relative contribution and importance of aggregations to fisheries production:
- - peak catch and effort during spawning season?
- Catchability increases during spawning season due to:
- - recreational or commercial targeting of aggregations
- - gear efficiency (e.g. flounder gigging)
- - regulations or lack thereof