Are Fish Really “Heading South”? The Real Reason Stocks Are Shifting — And It’s Not What Most People Think

Guest post by Andi Cockroft, Chair, CORANZ

Prompted by this RNZ article I decided to write this short note:

Public debate has recently been stirred by claims that marine fish are “moving south” as northern waters warm. The suggestion is often that adult fish are packing up en masse and swimming into cooler South Island waters. The truth, however, is considerably more nuanced — and far more important for long-term fisheries management.

For many of the species important to New Zealanders, such as snapper, trevally, gurnard, kahawai and blue cod, adults do not undertake long-distance migrations in response to short-term changes in sea temperature. Their daily and seasonal movements are relatively fixed. Even when northern waters run hotter than usual, adults tend to stay within familiar grounds unless pushed by more dramatic environmental changes.

So why do catch statistics, sightings and anecdotal reports increasingly point to expanding or strengthening populations further south?

The answer lies not in where the adults go, but in where the young survive.

Breeding Success — The Hidden Force Behind “Shifting Species”

Fish spawn in vast numbers, releasing eggs and larvae that are dispersed widely by ocean currents. These early life stages are extremely sensitive to temperature. Even small departures from their optimal range — hotter or colder — can drastically alter survival chances.

In the northern regions of New Zealand, repeated warm summers and occasional marine heatwaves have produced conditions that exceed the comfortable limits for the eggs and larvae of certain species. Temperatures 2–4 °C above historic norms have been recorded during extended marine heatwave events in the Tasman and northern waters. For many species, this reduces early survival rates, meaning fewer juveniles reach adulthood.

Meanwhile, waters further south — Cook Strait, the upper South Island, and even down the east coast — still sit closer to the ideal temperature range for early development. As a result, larval survival is greater further south than it used to be, even though spawning adults may still be distributed widely across regions.

Over multiple seasons, this pattern compounds. You end up with more young fish successfully growing up in southern areas, while fewer survive in the north. The adult population’s “centre of gravity” therefore shifts — not because adults travelled there, but because that is where the next generation happened to survive best.

This process is known internationally as a recruitment shift, and it explains most climate-linked changes in fish distribution observed around the world, from northern hemisphere cod to Mediterranean anchovies.

Why This Distinction Matters for New Zealand

If we assume fish are simply “swimming south”, we risk misunderstanding what is actually happening beneath the surface. The real drivers are:

• Larval survival rates, not adult migration
  
• Changes in temperature during spawning and early development
  
• Shifts in currents and heatwave frequency
  
• And, crucially, existing stressors such as habitat degradation and overfishing in northern regions
  

Northern stock depletion from decades of heavy fishing means any reduction in juvenile survival hits harder there. In contrast, southern waters — cooler and sometimes less pressured — can produce stronger year-classes.

This gives the illusion of movement, even though the underlying dynamic is about where young fish can thrive.

Implications for Fisheries Management

Understanding the true driver of these changes is essential if New Zealand is going to manage fish stocks responsibly in the future.

If recruitment is shifting south:

• Stock assessments must reflect regional breeding success, not just adult abundance.
  
• Pressure on vulnerable northern fisheries cannot be waved away by pointing to increases elsewhere.
  
• Southern waters may experience increased fishing pressure — recreational and commercial — if these new trends continue.
  
• Management needs to consider early-life conditions, not just adult populations.
  

For recreational fishers, it means recognising that future abundance patterns will depend heavily on protecting nursery areas, ensuring habitat quality, and reducing fishing pressure in regions where recruitment is faltering.

A More Accurate Narrative

Instead of saying “fish are heading south,” the more accurate picture is:

“More fish are growing up further south because that is where conditions now favour survival of juveniles.”

That may seem like a subtle difference, but it changes the entire management conversation. Adult fish wandering south is a short-term curiosity. Recruitment shifts are a long-term change with real consequences for fisheries, ecosystems, and communities.

As freshwater and coastal advocates, NZFFA members know that ecological change is rarely simple. But if we focus on the true biological mechanisms, not the headlines, we are far better positioned to ensure sustainable fisheries for the generations who come after us.