By Dr Jade Maggs, Marine Scientist…
In 1982, the English punk rock band, The Clash, released their hit song “Should I stay or should I go”, expressing indecision with which most of us are familiar. Staying or moving is essentially a trade-off decision. Staying in a place provides familiarity with surroundings, but it has always been human nature to wonder whether the grass is greener on the other side?
The song by The Clash goes on to say, “If I go, there will be trouble and if I stay, it will be double”. The decision to stay or move ultimately comes down to an evaluation of the positives and negatives of staying versus the positives and negatives of moving.
Within the human species, there are those that stay and those that go. Recent research has shown that this phenomenon is not unique to humans, but inherent in many species, especially birds and fish. Ecologists refer to this phenomenon as partial migration.
Migratory and resident fish species
Each year with the onset of winter, around May and June in South Africa, shore anglers in the province of KwaZulu-Natal eagerly await the arrival of migratory species, such as sardine Sardinops sagax, elf/shad Pomatomus saltatrix and leervis/garrick Lichia amia.
Along this coastline, boat anglers similarly anticipate the arrival of large dusky kob Argyrosomus japonicus and geelbek Atractoscion aequidens. Most of these fish have moved up the east coast of southern Africa from the Cape to spawn and are well-known as migratory species. They have predictable, seemingly coordinated, movement strategies covering vast distances.
On the other hand, we know that some species, such as galjoen Dichistius capensis and speckled snapper Lutjanus rivulatus are not migratory. These resident species occur in the same place year round throughout their distribution and show no marked seasonal shifts like migratory species.
Migratory residents and resident migrants
Despite the well-defined, coordinated movements of migratory or wide-ranging species and the site-attachment of resident species, it is becoming apparent that within each species there are non-conformists.
Even in well-known migratory species, some individuals remain resident throughout the year, not participating in the annual migration. For example, telemetry research conducted on elf in Langebaan Lagoon on the Cape West Coast of South Africa showed that some individuals, tagged with acoustic transmitters, were detected in the lagoon almost continuously over a 30-month period. This research shows that they did not participate in the annual migration.
There are also an increasing number of tag-recapture observations where individuals of resident species undertake long-distance movements. For example, a galjoen tagged at Lekkerwater in the De Hoop Marine Reserve moved 1320 km up the east coast and was recaptured just south of Durban after 130 days. Similarly, a slinger Chrysoblephus puniceus tagged in the Pondoland Marine Protected Area was recaptured off Quissico in Mozambique, more than 1000 km after 582 days. There are many similar examples.
Fishery scientists in southern Africa have been studying movement of fishes for the past nine decades (1928-2017). While this has taught us much about fish movement, there is still much to be learned about those individuals that do not conform to the regular pattern. In the past, these non-conformists received little attention and were often treated as unexplained anomalies or outliers.
But, these observations raise important questions. Why stay put when the rest of your population has migrated so far away? Why undertake a long-distance movement when the rest of your species is characterised by residency? Is this non-conformist behaviour persistent throughout the individual’s lifetime? Does this uncharacteristic behaviour benefit the individual or the population?
These questions are especially relevant to the placement of marine protected areas (MPAs).
Partial migration case study in southern Africa
As part of my PhD research, I investigated the movements of five South African fish species – garrick, raggedtooth shark Carcharias taurus, spotted gulleyshark Triakis megalopterus, galjoen and speckled snapper. I used tag-recapture data collected by anglers participating in the Oceanographic Research Institute Cooperative Fish Tagging Project (ORI-CFTP).
Using these data, I found evidence of residence and wide-ranging behaviour in juveniles and adults of all five species. There was also some indication that wide-ranging individuals grew faster, either because wide-ranging individuals found more food and therefore grew faster or faster growing individuals were encouraged to move to find more food resources.
But, what does this mean for the species and what does it mean for anglers? Research in other parts of the world suggests that the coexistence of residence and migration within a species provides resilience for the population. Residence is a low risk, low reward behaviour, while migration is a high risk, high reward behaviour.
This combination of strategies can be likened to a balanced investment portfolio. Resident individuals represent the steady, safe, slow-growth component of the portfolio, while migratory individuals are the high-risk, potentially high-reward component. In years when wide-ranging behaviour does not pay off, residents provide insurance for the population. However, in years favouring migration, migrants are able to take advantage of opportunities further afield and contribute many juveniles to the population.
So, this strategy provides resilience against population declines. It therefore also offers some protection against over-exploitation by anglers. However, this does not mean that our fish are immune to over-exploitation. It simply means that, without this strategy some of our over-exploited fish species would possibly be in worse shape if there was no variability in their movement behaviour. It also serves to highlight the importance of Marine Protected Areas in providing protection for the resident components of these fish populations.
I would like to thank the many participants in the ORI-CFTP, who have contributed their tagging data over the years. My PhD research was heavily dependent on your valuable contributions. The South African Association for Marine Biological Research and the South African Institute for Aquatic Biodiversity also provided substantial support for this research, which was conducted through the Department of Ichthyology and Fisheries Science at Rhodes University.