El Niño Winters and San Diego Hull Fouling: What Changes?

During El Niño winters, San Diego Bay water can run 6 to 10 degrees warmer than a normal year, sometimes sitting at 64-68°F in January instead of the usual 56-60°F. That's warm enough for slime and barnacle growth to keep going through winter instead of slowing down like it should. I've seen hulls that would normally coast through December and January on a quarterly schedule need attention every 6 weeks instead.

I've been diving this bay long enough to remember the last couple of strong El Niño winters, and the pattern is the same every time. Boat owners relax their cleaning schedule because it's winter, and then get surprised in February with a hull that looks like August.

Key takeaways

• El Niño can push bay water temps to 64-68°F in winter, compared to a normal 56-60°F
• Barnacle and tube worm growth barely slow down above 60°F, so winter isn't the "break" it usually is
• Boats on a quarterly plan are the most likely to get caught off guard
• Zinc anodes can also deplete faster in warmer, more active water, so free zinc checks matter more this season
• Shelter Island's copper TMDL zone rules still apply year round regardless of water temp

Why does warm winter water matter for fouling?

Marine growth in San Diego Bay slows down mainly because of two things: shorter daylight and colder water. Cold water is the bigger factor for most species. Barnacle larvae, tube worms, and the algae that coats a hull all settle and grow faster once water gets above roughly 60°F.

In a normal winter, bay temps drop into the mid to high 50s, and that alone knocks growth down significantly. That's why quarterly cleaning works fine for a lot of boats from December through February. In an El Niño winter, the water doesn't cool off that much, sometimes staying near 65°F straight through, and growth just keeps going at a pace closer to what you'd see in June.

I check water temp on almost every dive out of habit, and during the last strong El Niño stretch I was seeing more soft growth and juvenile barnacles on boats in January than I normally see until spring.

How much faster does fouling actually build up?

On a typical winter, a hull cleaned in November might still look decent by late January. During an El Niño winter, I've pulled boats out of the water (well, kept them in the water, since we don't haul out) after 8 weeks and found growth that should have taken 12-14 weeks to form.

Props and running gear are usually the first place it shows. Growth on props cuts efficiency and burns more fuel long before the hull itself looks bad. If your boat feels sluggish or you notice the engine working harder at the same RPM, that's often the first sign, even in what should be the slow season.

Should I change my cleaning schedule during an El Niño winter?

If you're on a quarterly plan, I'd bump to bi-monthly for the winter months in an El Niño year. If you're already on monthly, stick with it, that interval holds up regardless of water temp. The boats that get hit hardest are the ones on the widest intervals, because there's more time for warm water to do its work between visits.

This doesn't need to be permanent. Once the pattern breaks and water temps drop back to normal (usually by March or April even in a strong El Niño year), you can go back to your regular schedule. I'll flag it when I'm diving your boat if I think the interval needs adjusting, and it costs nothing extra since it's just a scheduling change.

Does this affect zinc anodes too?

Yes. Warmer water generally means more electrical activity in the water and can speed up zinc consumption, especially paired with the increased biological activity around fittings and shafts. I check zinc condition on every single dive at no charge, so if consumption is running faster than normal, you'll know before it becomes a problem. Replacement is cost-plus on the part itself, no markup, so there's no reason to put it off if a zinc is getting thin.

FAQ

Does El Niño affect the whole bay the same way, or just certain areas? It affects the whole bay, but shallower, more enclosed areas like Mission Bay and parts of Chula Vista tend to warm up a bit more than open channel areas near Point Loma. Shelter Island and Harbor Island sit somewhere in the middle.

How do I know if this winter is an El Niño pattern? NOAA tracks this through their Climate Prediction Center, and it's usually reported on well before winter starts. If you're not sure, just ask me when I'm doing your cleaning. I keep an eye on it because it affects how I schedule my own route.

Will my antifouling paint wear out faster in an El Niño winter? Not directly from the warm water itself, but more frequent cleanings to keep up with growth means more contact with the hull. I use the soft-cloth BMP standard on every job specifically to protect the paint, so extra visits shouldn't accelerate wear if it's done right.

Is this only a winter issue, or does El Niño affect summer fouling too? Summer fouling in San Diego Bay is already at its peak regardless of El Niño, so the effect is much less noticeable then. The real difference shows up in winter, when growth should be slowing down and instead keeps chugging along.

Do I need a permit for more frequent in-water cleanings? No extra permit is needed for additional visits. The San Diego In-Water Hull Cleaning Permit covers the activity itself, not the frequency, and I hold that permit and follow the rules on every dive regardless of how often I'm out there.

Keep ahead of it this winter

If you're not sure whether your boat needs a schedule adjustment this season, that's an easy thing to figure out on the next dive. I'll check growth, water clarity, and zinc condition and give you a straight answer about whether quarterly is still working or if it's time to tighten up the interval. No pressure either way. Get a quote or book a hull cleaning here.

SCHEMA NOTES

FAQPage Q&As: 1. Q: Does El Niño affect the whole bay the same way, or just certain areas? A: It affects the whole bay, but shallower, more enclosed areas like Mission Bay and parts of Chula Vista tend to warm up a bit more than open channel areas near Point Loma. Shelter Island and Harbor Island sit somewhere in the middle. 2. Q: How do I know if this winter is an El Niño pattern? A: NOAA tracks this through their Climate Prediction Center, and it's usually reported on well before winter starts. If you're not sure, just ask me when I'm doing your cleaning. 3. Q: Will my antifouling paint wear out faster in an El Niño winter? A: Not directly from the warm water itself, but more frequent cleanings to keep up with growth means more contact with the hull. I use the soft-cloth BMP standard on every job specifically to protect the paint. 4. Q: Is this only a winter issue, or does El Niño affect summer fouling too? A: Summer fouling in San Diego Bay is already at its peak regardless of El Niño, so the effect is much less noticeable then. The real difference shows up in winter. 5. Q: Do I need a permit for more frequent in-water cleanings? A: No extra permit is needed for additional visits. The San Diego In-Water Hull Cleaning Permit covers the activity itself, and I hold that permit and follow the rules on every dive.

BlogPosting summary: Explains how El Niño winters warm San Diego Bay enough to keep hull fouling active through winter months, and what boat owners should adjust in their cleaning schedule as a result.

Suggested images: - Alt: "Diver inspecting a hull for winter marine growth in San Diego Bay during an El Niño year" - Alt: "Close-up of barnacle growth on a boat propeller in warmer than normal winter bay water"